Writing SELECT statements for Inheritance Mappings

ORM Querying Guide

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Writing SELECT statements for Inheritance Mappings

About this Document

This section makes use of ORM mappings configured using the ORM Inheritance feature, described at Mapping Class Inheritance Hierarchies. The emphasis will be on Joined Table Inheritance as this is the most intricate ORM querying case.

View the ORM setup for this page.

SELECTing from the base class vs. specific sub-classes

A SELECT statement constructed against a class in a joined inheritance hierarchy will query against the table to which the class is mapped, as well as any super-tables present, using JOIN to link them together. The query would then return objects that are of that requested type as well as any sub-types of the requested type, using the discriminator value in each row to determine the correct type. The query below is established against the Manager subclass of Employee, which then returns a result that will contain only objects of type Manager:

>>> from sqlalchemy import select
>>> stmt = select(Manager).order_by(Manager.id)
>>> managers = session.scalars(stmt).all()
BEGIN (implicit)
SELECT manager.id, employee.id AS id_1, employee.name, employee.type, employee.company_id, manager.manager_name
FROM employee JOIN manager ON employee.id = manager.id ORDER BY manager.id
[...] ()
>>> print(managers)
[Manager('Mr. Krabs')]

When the SELECT statement is against the base class in the hierarchy, the default behavior is that only that class’ table will be included in the rendered SQL and JOIN will not be used. As in all cases, the discriminator column is used to distinguish between different requested sub-types, which then results in objects of any possible sub-type being returned. The objects returned will have attributes corresponding to the base table populated, and attributes corresponding to sub-tables will start in an un-loaded state, loading automatically when accessed. The loading of sub-attributes is configurable to be more “eager” in a variety of ways, discussed later in this section.

The example below creates a query against the Employee superclass. This indicates that objects of any type, including Manager, Engineer, and Employee, may be within the result set:

>>> from sqlalchemy import select
>>> stmt = select(Employee).order_by(Employee.id)
>>> objects = session.scalars(stmt).all()
BEGIN (implicit)
SELECT employee.id, employee.name, employee.type, employee.company_id
FROM employee ORDER BY employee.id
[...] ()
>>> print(objects)
[Manager('Mr. Krabs'), Engineer('SpongeBob'), Engineer('Squidward')]

Above, the additional tables for Manager and Engineer were not included in the SELECT, which means that the returned objects will not yet contain data represented from those tables, in this example the .manager_name attribute of the Manager class as well as the .engineer_info attribute of the Engineer class. These attributes start out in the expired state, and will automatically populate themselves when first accessed using lazy loading:

>>> mr_krabs = objects[0]
>>> print(mr_krabs.manager_name)
SELECT manager.manager_name AS manager_manager_name
FROM manager
WHERE ? = manager.id
[...] (1,)
Eugene H. Krabs

This lazy load behavior is not desirable if a large number of objects have been loaded, in the case that the consuming application will need to be accessing subclass-specific attributes, as this would be an example of the N plus one problem that emits additional SQL per row. This additional SQL can impact performance and also be incompatible with approaches such as using asyncio. Additionally, in our query for Employee objects, since the query is against the base table only, we did not have a way to add SQL criteria involving subclass-specific attributes in terms of Manager or Engineer. The next two sections detail two constructs that provide solutions to these two issues in different ways, the selectin_polymorphic() loader option and the with_polymorphic() entity construct.

Using selectin_polymorphic()

To address the issue of performance when accessing attributes on subclasses, the selectin_polymorphic() loader strategy may be used to eagerly load these additional attributes up front across many objects at once. This loader option works in a similar fashion as the selectinload() relationship loader strategy to emit an additional SELECT statement against each sub-table for objects loaded in the hierarchy, using IN to query for additional rows based on primary key.

selectinload() accepts as its arguments the base entity that is being queried, followed by a sequence of subclasses of that entity for which their specific attributes should be loaded for incoming rows:

>>> from sqlalchemy.orm import selectin_polymorphic
>>> loader_opt = selectin_polymorphic(Employee, [Manager, Engineer])

The selectin_polymorphic() construct is then used as a loader option, passing it to the Select.options() method of Select. The example illustrates the use of selectin_polymorphic() to eagerly load columns local to both the Manager and Engineer subclasses:

>>> from sqlalchemy.orm import selectin_polymorphic
>>> loader_opt = selectin_polymorphic(Employee, [Manager, Engineer])
>>> stmt = select(Employee).order_by(Employee.id).options(loader_opt)
>>> objects = session.scalars(stmt).all()
BEGIN (implicit)
SELECT employee.id, employee.name, employee.type, employee.company_id
FROM employee ORDER BY employee.id
[...] ()
SELECT manager.id AS manager_id, employee.id AS employee_id,
employee.type AS employee_type, manager.manager_name AS manager_manager_name
FROM employee JOIN manager ON employee.id = manager.id
WHERE employee.id IN (?) ORDER BY employee.id
[...] (1,)
SELECT engineer.id AS engineer_id, employee.id AS employee_id,
employee.type AS employee_type, engineer.engineer_info AS engineer_engineer_info
FROM employee JOIN engineer ON employee.id = engineer.id
WHERE employee.id IN (?, ?) ORDER BY employee.id
[...] (2, 3)
>>> print(objects)
[Manager('Mr. Krabs'), Engineer('SpongeBob'), Engineer('Squidward')]

The above example illustrates two additional SELECT statements being emitted in order to eagerly fetch additional attributes such as Engineer.engineer_info as well as Manager.manager_name. We can now access these sub-attributes on the objects that were loaded without any additional SQL statements being emitted:

>>> print(objects[0].manager_name)
Eugene H. Krabs

Tip

The selectin_polymorphic() loader option does not yet optimize for the fact that the base employee table does not need to be included in the second two “eager load” queries; hence in the example above we see a JOIN from employee to manager and engineer, even though columns from employee are already loaded. This is in contrast to the selectinload() relationship strategy which is more sophisticated in this regard and can factor out the JOIN when not needed.

Combining additional loader options with selectin_polymorphic() subclass loads

The SELECT statements emitted by selectin_polymorphic() are themselves ORM statements, so we may also add other loader options (such as those documented at Relationship Loading Techniques) that refer to specific subclasses. For example, if we considered that the Manager mapper had a one to many relationship to an entity called Paperwork, we could combine the use of selectin_polymorphic() and selectinload() to eagerly load this collection on all Manager objects, where the sub-attributes of Manager objects were also themselves eagerly loaded:

>>> from sqlalchemy.orm import selectinload
>>> from sqlalchemy.orm import selectin_polymorphic
>>> stmt = (
...     select(Employee)
...     .order_by(Employee.id)
...     .options(
...         selectin_polymorphic(Employee, [Manager, Engineer]),
...         selectinload(Manager.paperwork),
...     )
... )
>>> objects = session.scalars(stmt).all()
BEGIN (implicit)
SELECT employee.id, employee.name, employee.type, employee.company_id
FROM employee ORDER BY employee.id
[...] ()
SELECT manager.id AS manager_id, employee.id AS employee_id, employee.type AS employee_type, manager.manager_name AS manager_manager_name
FROM employee JOIN manager ON employee.id = manager.id
WHERE employee.id IN (?) ORDER BY employee.id
[...] (1,)
SELECT paperwork.manager_id AS paperwork_manager_id, paperwork.id AS paperwork_id, paperwork.document_name AS paperwork_document_name
FROM paperwork
WHERE paperwork.manager_id IN (?)
[...] (1,)
SELECT engineer.id AS engineer_id, employee.id AS employee_id, employee.type AS employee_type, engineer.engineer_info AS engineer_engineer_info
FROM employee JOIN engineer ON employee.id = engineer.id
WHERE employee.id IN (?, ?) ORDER BY employee.id
[...] (2, 3)
>>> print(objects[0])
Manager('Mr. Krabs')
>>> print(objects[0].paperwork)
[Paperwork('Secret Recipes'), Paperwork('Krabby Patty Orders')]

Applying selectin_polymorphic() to an existing eager load

In addition to being able to add loader options to the right side of a selectin_polymorphic() load, we may also indicate selectin_polymorphic() on the target of an existing load. As our setup mapping includes a parent Company entity with a Company.employees relationship() referring to Employee entities, we may illustrate a SELECT against the Company entity that eagerly loads all Employee objects as well as all attributes on their subtypes as follows, by applying Load.selectin_polymorphic() as a chained loader option; in this form, the first argument is implicit from the previous loader option (in this case selectinload()), so we only indicate the additional target subclasses we wish to load:

>>> stmt = select(Company).options(
...     selectinload(Company.employees).selectin_polymorphic([Manager, Engineer])
... )
>>> for company in session.scalars(stmt):
...     print(f"company: {company.name}")
...     print(f"employees: {company.employees}")
SELECT company.id, company.name
FROM company
[...] ()
SELECT employee.company_id AS employee_company_id, employee.id AS employee_id,
employee.name AS employee_name, employee.type AS employee_type
FROM employee
WHERE employee.company_id IN (?)
[...] (1,)
SELECT manager.id AS manager_id, employee.id AS employee_id, employee.name AS employee_name,
employee.type AS employee_type, employee.company_id AS employee_company_id,
manager.manager_name AS manager_manager_name
FROM employee JOIN manager ON employee.id = manager.id
WHERE employee.id IN (?) ORDER BY employee.id
[...] (1,)
SELECT engineer.id AS engineer_id, employee.id AS employee_id, employee.name AS employee_name,
employee.type AS employee_type, employee.company_id AS employee_company_id,
engineer.engineer_info AS engineer_engineer_info
FROM employee JOIN engineer ON employee.id = engineer.id
WHERE employee.id IN (?, ?) ORDER BY employee.id
[...] (2, 3)
company: Krusty Krab
employees: [Manager('Mr. Krabs'), Engineer('SpongeBob'), Engineer('Squidward')]

See also

Eager Loading of Polymorphic Subtypes - illustrates the equivalent example as above using with_polymorphic() instead

Configuring selectin_polymorphic() on mappers

The behavior of selectin_polymorphic() may be configured on specific mappers so that it takes place by default, by using the Mapper.polymorphic_load parameter, using the value "selectin" on a per-subclass basis. The example below illustrates the use of this parameter within Engineer and Manager subclasses:

class Employee(Base):
    __tablename__ = "employee"
    id = mapped_column(Integer, primary_key=True)
    name = mapped_column(String(50))
    type = mapped_column(String(50))
    __mapper_args__ = {"polymorphic_identity": "employee", "polymorphic_on": type}
class Engineer(Employee):
    __tablename__ = "engineer"
    id = mapped_column(Integer, ForeignKey("employee.id"), primary_key=True)
    engineer_info = mapped_column(String(30))
    __mapper_args__ = {
        "polymorphic_load": "selectin",
        "polymorphic_identity": "engineer",
    }
class Manager(Employee):
    __tablename__ = "manager"
    id = mapped_column(Integer, ForeignKey("employee.id"), primary_key=True)
    manager_name = mapped_column(String(30))
    __mapper_args__ = {
        "polymorphic_load": "selectin",
        "polymorphic_identity": "manager",
    }

With the above mapping, SELECT statements against the Employee class will automatically assume the use of selectin_polymorphic(Employee, [Engineer, Manager]) as a loader option when the statement is emitted.

Using with_polymorphic()

In contrast to selectin_polymorphic() which affects only the loading of objects, the with_polymorphic() construct affects how the SQL query for a polymorphic structure is rendered, most commonly as a series of LEFT OUTER JOINs to each of the included sub-tables. This join structure is referred towards as the polymorphic selectable. By providing for a view of several sub-tables at once, with_polymorphic() offers a means of writing a SELECT statement across several inherited classes at once with the ability to add filtering criteria based on individual sub-tables.

with_polymorphic() is essentially a special form of the aliased() construct. It accepts as its arguments a similar form to that of selectin_polymorphic(), which is the base entity that is being queried, followed by a sequence of subclasses of that entity for which their specific attributes should be loaded for incoming rows:

>>> from sqlalchemy.orm import with_polymorphic
>>> employee_poly = with_polymorphic(Employee, [Engineer, Manager])

In order to indicate that all subclasses should be part of the entity, with_polymorphic() will also accept the string "*", which may be passed in place of the sequence of classes to indicate all classes (note this is not yet supported by selectin_polymorphic()):

>>> employee_poly = with_polymorphic(Employee, "*")

The example below illustrates the same operation as illustrated in the previous section, to load all columns for Manager and Engineer at once:

>>> stmt = select(employee_poly).order_by(employee_poly.id)
>>> objects = session.scalars(stmt).all()
BEGIN (implicit)
SELECT employee.id, employee.name, employee.type, employee.company_id,
manager.id AS id_1, manager.manager_name, engineer.id AS id_2, engineer.engineer_info
FROM employee
LEFT OUTER JOIN manager ON employee.id = manager.id
LEFT OUTER JOIN engineer ON employee.id = engineer.id ORDER BY employee.id
[...] ()
>>> print(objects)
[Manager('Mr. Krabs'), Engineer('SpongeBob'), Engineer('Squidward')]

As is the case with selectin_polymorphic(), attributes on subclasses are already loaded:

>>> print(objects[0].manager_name)
Eugene H. Krabs

As the default selectable produced by with_polymorphic() uses LEFT OUTER JOIN, from a database point of view the query is not as well optimized as the approach that selectin_polymorphic() takes, with simple SELECT statements using only JOINs emitted on a per-table basis.

Filtering Subclass Attributes with with_polymorphic()

The with_polymorphic() construct makes available the attributes on the included subclass mappers, by including namespaces that allow references to subclasses. The employee_poly construct created in the previous section includes attributes named .Engineer and .Manager which provide the namespace for Engineer and Manager in terms of the polymorphic SELECT. In the example below, we can use the or_() construct to create criteria against both classes at once:

>>> from sqlalchemy import or_
>>> employee_poly = with_polymorphic(Employee, [Engineer, Manager])
>>> stmt = (
...     select(employee_poly)
...     .where(
...         or_(
...             employee_poly.Manager.manager_name == "Eugene H. Krabs",
...             employee_poly.Engineer.engineer_info
...             == "Senior Customer Engagement Engineer",
...         )
...     )
...     .order_by(employee_poly.id)
... )
>>> objects = session.scalars(stmt).all()
SELECT employee.id, employee.name, employee.type, employee.company_id, manager.id AS id_1,
manager.manager_name, engineer.id AS id_2, engineer.engineer_info
FROM employee
LEFT OUTER JOIN manager ON employee.id = manager.id
LEFT OUTER JOIN engineer ON employee.id = engineer.id
WHERE manager.manager_name = ? OR engineer.engineer_info = ?
ORDER BY employee.id
[...] ('Eugene H. Krabs', 'Senior Customer Engagement Engineer')
>>> print(objects)
[Manager('Mr. Krabs'), Engineer('Squidward')]

Using aliasing with with_polymorphic

The with_polymorphic() construct, as a special case of aliased(), also provides the basic feature that aliased() does, which is that of “aliasing” of the polymorphic selectable itself. Specifically this means two or more with_polymorphic() entities, referring to the same class hierarchy, can be used at once in a single statement.

To use this feature with a joined inheritance mapping, we typically want to pass two parameters, with_polymorphic.aliased as well as with_polymorphic.flat. The with_polymorphic.aliased parameter indicates that the polymorphic selectable should be referred towards by an alias name that is unique to this construct. The with_polymorphic.flat parameter is specific to the default LEFT OUTER JOIN polymorphic selectable and indicates that a more optimized form of aliasing should be used in the statement.

To illustrate this feature, the example below emits a SELECT for two separate polymorphic entities, Employee joined with Engineer, and Employee joined with Manager. Since these two polymorphic entities will both be including the base employee table in their polymorphic selectable, aliasing must be applied in order to differentiate this table in its two different contexts. The two polymorphic entities are treated like two individual tables, and as such typically need to be joined with each other in some way, as illustrated below where the entities are joined on the company_id column along with some additional limiting criteria against the Employee / Manager entity:

>>> manager_employee = with_polymorphic(Employee, [Manager], aliased=True, flat=True)
>>> engineer_employee = with_polymorphic(Employee, [Engineer], aliased=True, flat=True)
>>> stmt = (
...     select(manager_employee, engineer_employee)
...     .join(
...         engineer_employee,
...         engineer_employee.company_id == manager_employee.company_id,
...     )
...     .where(
...         or_(
...             manager_employee.name == "Mr. Krabs",
...             manager_employee.Manager.manager_name == "Eugene H. Krabs",
...         )
...     )
...     .order_by(engineer_employee.name, manager_employee.name)
... )
>>> for manager, engineer in session.execute(stmt):
...     print(f"{manager} {engineer}")
SELECT
employee_1.id, employee_1.name, employee_1.type, employee_1.company_id,
manager_1.id AS id_1, manager_1.manager_name,
employee_2.id AS id_2, employee_2.name AS name_1, employee_2.type AS type_1,
employee_2.company_id AS company_id_1, engineer_1.id AS id_3, engineer_1.engineer_info
FROM employee AS employee_1
LEFT OUTER JOIN manager AS manager_1 ON employee_1.id = manager_1.id
JOIN
   (employee AS employee_2 LEFT OUTER JOIN engineer AS engineer_1 ON employee_2.id = engineer_1.id)
ON employee_2.company_id = employee_1.company_id
WHERE employee_1.name = ? OR manager_1.manager_name = ?
ORDER BY employee_2.name, employee_1.name
[...] ('Mr. Krabs', 'Eugene H. Krabs')
Manager('Mr. Krabs') Manager('Mr. Krabs')
Manager('Mr. Krabs') Engineer('SpongeBob')
Manager('Mr. Krabs') Engineer('Squidward')

In the above example, the behavior of with_polymorphic.flat is that the polymorphic selectables remain as a LEFT OUTER JOIN of their individual tables, which themselves are given anonymous alias names. There is also a right-nested JOIN produced.

When omitting the with_polymorphic.flat parameter, the usual behavior is that each polymorphic selectable is enclosed within a subquery, producing a more verbose form:

>>> manager_employee = with_polymorphic(Employee, [Manager], aliased=True)
>>> engineer_employee = with_polymorphic(Employee, [Engineer], aliased=True)
>>> stmt = (
...     select(manager_employee, engineer_employee)
...     .join(
...         engineer_employee,
...         engineer_employee.company_id == manager_employee.company_id,
...     )
...     .where(
...         or_(
...             manager_employee.name == "Mr. Krabs",
...             manager_employee.Manager.manager_name == "Eugene H. Krabs",
...         )
...     )
...     .order_by(engineer_employee.name, manager_employee.name)
... )
>>> print(stmt)
SELECT anon_1.employee_id, anon_1.employee_name, anon_1.employee_type,
anon_1.employee_company_id, anon_1.manager_id, anon_1.manager_manager_name, anon_2.employee_id AS employee_id_1,
anon_2.employee_name AS employee_name_1, anon_2.employee_type AS employee_type_1,
anon_2.employee_company_id AS employee_company_id_1, anon_2.engineer_id, anon_2.engineer_engineer_info
FROM
(SELECT employee.id AS employee_id, employee.name AS employee_name, employee.type AS employee_type,
employee.company_id AS employee_company_id,
manager.id AS manager_id, manager.manager_name AS manager_manager_name
FROM employee LEFT OUTER JOIN manager ON employee.id = manager.id) AS anon_1
JOIN
(SELECT employee.id AS employee_id, employee.name AS employee_name, employee.type AS employee_type,
employee.company_id AS employee_company_id, engineer.id AS engineer_id, engineer.engineer_info AS engineer_engineer_info
FROM employee LEFT OUTER JOIN engineer ON employee.id = engineer.id) AS anon_2
ON anon_2.employee_company_id = anon_1.employee_company_id
WHERE anon_1.employee_name = :employee_name_2 OR anon_1.manager_manager_name = :manager_manager_name_1
ORDER BY anon_2.employee_name, anon_1.employee_name

The above form historically has been more portable to backends that didn’t necessarily have support for right-nested JOINs, and it additionally may be appropriate when the “polymorphic selectable” used by with_polymorphic() is not a simple LEFT OUTER JOIN of tables, as is the case when using mappings such as concrete table inheritance mappings as well as when using alternative polymorphic selectables in general.

Configuring with_polymorphic() on mappers

As is the case with selectin_polymorphic(), the with_polymorphic() construct also supports a mapper-configured version which may be configured in two different ways, either on the base class using the mapper.with_polymorphic parameter, or in a more modern form using the Mapper.polymorphic_load parameter on a per-subclass basis, passing the value "inline".

Warning

For joined inheritance mappings, prefer explicit use of with_polymorphic() within queries, or for implicit eager subclass loading use Mapper.polymorphic_load with "selectin", instead of using the mapper-level mapper.with_polymorphic parameter described in this section. This parameter invokes complex heuristics intended to rewrite the FROM clauses within SELECT statements that can interfere with construction of more complex statements, particularly those with nested subqueries that refer to the same mapped entity.

For example, we may state our Employee mapping using Mapper.polymorphic_load as "inline" as below:

class Employee(Base):
    __tablename__ = "employee"
    id = mapped_column(Integer, primary_key=True)
    name = mapped_column(String(50))
    type = mapped_column(String(50))
    __mapper_args__ = {"polymorphic_identity": "employee", "polymorphic_on": type}
class Engineer(Employee):
    __tablename__ = "engineer"
    id = mapped_column(Integer, ForeignKey("employee.id"), primary_key=True)
    engineer_info = mapped_column(String(30))
    __mapper_args__ = {
        "polymorphic_load": "inline",
        "polymorphic_identity": "engineer",
    }
class Manager(Employee):
    __tablename__ = "manager"
    id = mapped_column(Integer, ForeignKey("employee.id"), primary_key=True)
    manager_name = mapped_column(String(30))
    __mapper_args__ = {
        "polymorphic_load": "inline",
        "polymorphic_identity": "manager",
    }

With the above mapping, SELECT statements against the Employee class will automatically assume the use of with_polymorphic(Employee, [Engineer, Manager]) as the primary entity when the statement is emitted:

print(select(Employee))
SELECT employee.id, employee.name, employee.type, engineer.id AS id_1,
engineer.engineer_info, manager.id AS id_2, manager.manager_name
FROM employee
LEFT OUTER JOIN engineer ON employee.id = engineer.id
LEFT OUTER JOIN manager ON employee.id = manager.id

When using mapper-level “with polymorphic”, queries can also refer to the subclass entities directly, where they implicitly represent the joined tables in the polymorphic query. Above, we can freely refer to Manager and Engineer directly against the default Employee entity:

print(
    select(Employee).where(
        or_(Manager.manager_name == "x", Engineer.engineer_info == "y")
    )
)
SELECT employee.id, employee.name, employee.type, engineer.id AS id_1,
engineer.engineer_info, manager.id AS id_2, manager.manager_name
FROM employee
LEFT OUTER JOIN engineer ON employee.id = engineer.id
LEFT OUTER JOIN manager ON employee.id = manager.id
WHERE manager.manager_name = :manager_name_1
OR engineer.engineer_info = :engineer_info_1

However, if we needed to refer to the Employee entity or its sub entities in separate, aliased contexts, we would again make direct use of with_polymorphic() to define these aliased entities as illustrated in Using aliasing with with_polymorphic.

For more centralized control over the polymorphic selectable, the more legacy form of mapper-level polymorphic control may be used which is the Mapper.with_polymorphic parameter, configured on the base class. This parameter accepts arguments that are comparable to the with_polymorphic() construct, however common use with a joined inheritance mapping is the plain asterisk, indicating all sub-tables should be LEFT OUTER JOINED, as in:

class Employee(Base):
    __tablename__ = "employee"
    id = mapped_column(Integer, primary_key=True)
    name = mapped_column(String(50))
    type = mapped_column(String(50))
    __mapper_args__ = {
        "polymorphic_identity": "employee",
        "with_polymorphic": "*",
        "polymorphic_on": type,
    }
class Engineer(Employee):
    __tablename__ = "engineer"
    id = mapped_column(Integer, ForeignKey("employee.id"), primary_key=True)
    engineer_info = mapped_column(String(30))
    __mapper_args__ = {
        "polymorphic_identity": "engineer",
    }
class Manager(Employee):
    __tablename__ = "manager"
    id = mapped_column(Integer, ForeignKey("employee.id"), primary_key=True)
    manager_name = mapped_column(String(30))
    __mapper_args__ = {
        "polymorphic_identity": "manager",
    }

Overall, the LEFT OUTER JOIN format used by with_polymorphic() and by options such as Mapper.with_polymorphic may be cumbersome from a SQL and database optimizer point of view; for general loading of subclass attributes in joined inheritance mappings, the selectin_polymorphic() approach, or its mapper level equivalent of setting Mapper.polymorphic_load to "selectin" should likely be preferred, making use of with_polymorphic() on a per-query basis only as needed.

Joining to specific sub-types or with_polymorphic() entities

As a with_polymorphic() entity is a special case of aliased(), in order to treat a polymorphic entity as the target of a join, specifically when using a relationship() construct as the ON clause, we use the same technique for regular aliases as detailed at Using Relationship to join between aliased targets, most succinctly using PropComparator.of_type(). In the example below we illustrate a join from the parent Company entity along the one-to-many relationship Company.employees, which is configured in the setup to link to Employee objects, using a with_polymorphic() entity as the target:

>>> employee_plus_engineer = with_polymorphic(Employee, [Engineer])
>>> stmt = (
...     select(Company.name, employee_plus_engineer.name)
...     .join(Company.employees.of_type(employee_plus_engineer))
...     .where(
...         or_(
...             employee_plus_engineer.name == "SpongeBob",
...             employee_plus_engineer.Engineer.engineer_info
...             == "Senior Customer Engagement Engineer",
...         )
...     )
... )
>>> for company_name, emp_name in session.execute(stmt):
...     print(f"{company_name} {emp_name}")
SELECT company.name, employee.name AS name_1
FROM company JOIN (employee LEFT OUTER JOIN engineer ON employee.id = engineer.id) ON company.id = employee.company_id
WHERE employee.name = ? OR engineer.engineer_info = ?
[...] ('SpongeBob', 'Senior Customer Engagement Engineer')
Krusty Krab SpongeBob
Krusty Krab Squidward

More directly, PropComparator.of_type() is also used with inheritance mappings of any kind to limit a join along a relationship() to a particular sub-type of the relationship()’s target. The above query could be written strictly in terms of Engineer targets as follows:

>>> stmt = (
...     select(Company.name, Engineer.name)
...     .join(Company.employees.of_type(Engineer))
...     .where(
...         or_(
...             Engineer.name == "SpongeBob",
...             Engineer.engineer_info == "Senior Customer Engagement Engineer",
...         )
...     )
... )
>>> for company_name, emp_name in session.execute(stmt):
...     print(f"{company_name} {emp_name}")
SELECT company.name, employee.name AS name_1
FROM company JOIN (employee JOIN engineer ON employee.id = engineer.id) ON company.id = employee.company_id
WHERE employee.name = ? OR engineer.engineer_info = ?
[...] ('SpongeBob', 'Senior Customer Engagement Engineer')
Krusty Krab SpongeBob
Krusty Krab Squidward

It can be observed above that joining to the Engineer target directly, rather than the “polymorphic selectable” of with_polymorphic(Employee, [Engineer]) has the useful characteristic of using an inner JOIN rather than a LEFT OUTER JOIN, which is generally more performant from a SQL optimizer point of view.

Eager Loading of Polymorphic Subtypes

The use of PropComparator.of_type() illustrated with the Select.join() method in the previous section may also be applied equivalently to relationship loader options, such as selectinload() and joinedload().

As a basic example, if we wished to load Company objects, and additionally eagerly load all elements of Company.employees using the with_polymorphic() construct against the full hierarchy, we may write:

>>> all_employees = with_polymorphic(Employee, "*")
>>> stmt = select(Company).options(selectinload(Company.employees.of_type(all_employees)))
>>> for company in session.scalars(stmt):
...     print(f"company: {company.name}")
...     print(f"employees: {company.employees}")
SELECT company.id, company.name
FROM company
[...] ()
SELECT employee.company_id AS employee_company_id, employee.id AS employee_id,
employee.name AS employee_name, employee.type AS employee_type, manager.id AS manager_id,
manager.manager_name AS manager_manager_name, engineer.id AS engineer_id,
engineer.engineer_info AS engineer_engineer_info
FROM employee
LEFT OUTER JOIN manager ON employee.id = manager.id
LEFT OUTER JOIN engineer ON employee.id = engineer.id
WHERE employee.company_id IN (?)
[...] (1,)
company: Krusty Krab
employees: [Manager('Mr. Krabs'), Engineer('SpongeBob'), Engineer('Squidward')]

The above query may be compared directly to the selectin_polymorphic() version illustrated in the previous section Applying selectin_polymorphic() to an existing eager load.

See also

Applying selectin_polymorphic() to an existing eager load - illustrates the equivalent example as above using selectin_polymorphic() instead

SELECT Statements for Single Inheritance Mappings

Single Table Inheritance Setup

This section discusses single table inheritance, described at Single Table Inheritance, which uses a single table to represent multiple classes in a hierarchy.

View the ORM setup for this section.

In contrast to joined inheritance mappings, the construction of SELECT statements for single inheritance mappings tends to be simpler since for an all-single-inheritance hierarchy, there’s only one table.

Regardless of whether or not the inheritance hierarchy is all single-inheritance or has a mixture of joined and single inheritance, SELECT statements for single inheritance differentiate queries against the base class vs. a subclass by limiting the SELECT statement with additional WHERE criteria.

As an example, a query for the single-inheritance example mapping of Employee will load objects of type Manager, Engineer and Employee using a simple SELECT of the table:

>>> stmt = select(Employee).order_by(Employee.id)
>>> for obj in session.scalars(stmt):
...     print(f"{obj}")
BEGIN (implicit)
SELECT employee.id, employee.name, employee.type
FROM employee ORDER BY employee.id
[...] ()
Manager('Mr. Krabs')
Engineer('SpongeBob')
Engineer('Squidward')

When a load is emitted for a specific subclass, additional criteria is added to the SELECT that limits the rows, such as below where a SELECT against the Engineer entity is performed:

>>> stmt = select(Engineer).order_by(Engineer.id)
>>> objects = session.scalars(stmt).all()
SELECT employee.id, employee.name, employee.type, employee.engineer_info
FROM employee
WHERE employee.type IN (?) ORDER BY employee.id
[...] ('engineer',)
>>> for obj in objects:
...     print(f"{obj}")
Engineer('SpongeBob')
Engineer('Squidward')

Optimizing Attribute Loads for Single Inheritance

The default behavior of single inheritance mappings regarding how attributes on subclasses are SELECTed is similar to that of joined inheritance, in that subclass-specific attributes still emit a second SELECT by default. In the example below, a single Employee of type Manager is loaded, however since the requested class is Employee, the Manager.manager_name attribute is not present by default, and an additional SELECT is emitted when it’s accessed:

>>> mr_krabs = session.scalars(select(Employee).where(Employee.name == "Mr. Krabs")).one()
BEGIN (implicit)
SELECT employee.id, employee.name, employee.type
FROM employee
WHERE employee.name = ?
[...] ('Mr. Krabs',)
>>> mr_krabs.manager_name
SELECT employee.manager_name AS employee_manager_name
FROM employee
WHERE employee.id = ? AND employee.type IN (?)
[...] (1, 'manager')
'Eugene H. Krabs'

To alter this behavior, the same general concepts used to eagerly load these additional attributes used in joined inheritance loading apply to single inheritance as well, including use of the selectin_polymorphic() option as well as the with_polymorphic() option, the latter of which simply includes the additional columns and from a SQL perspective is more efficient for single-inheritance mappers:

>>> employees = with_polymorphic(Employee, "*")
>>> stmt = select(employees).order_by(employees.id)
>>> objects = session.scalars(stmt).all()
BEGIN (implicit)
SELECT employee.id, employee.name, employee.type,
employee.manager_name, employee.engineer_info
FROM employee ORDER BY employee.id
[...] ()
>>> for obj in objects:
...     print(f"{obj}")
Manager('Mr. Krabs')
Engineer('SpongeBob')
Engineer('Squidward')
>>> objects[0].manager_name
'Eugene H. Krabs'

Since the overhead of loading single-inheritance subclass mappings is usually minimal, it’s therefore recommended that single inheritance mappings include the Mapper.polymorphic_load parameter with a setting of "inline" for those subclasses where loading of their specific subclass attributes is expected to be common. An example illustrating the setup, modified to include this option, is below:

>>> class Base(DeclarativeBase):
...     pass
>>> class Employee(Base):
...     __tablename__ = "employee"
...     id: Mapped[int] = mapped_column(primary_key=True)
...     name: Mapped[str]
...     type: Mapped[str]
...
...     def __repr__(self):
...         return f"{self.__class__.__name__}({self.name!r})"
...
...     __mapper_args__ = {
...         "polymorphic_identity": "employee",
...         "polymorphic_on": "type",
...     }
>>> class Manager(Employee):
...     manager_name: Mapped[str] = mapped_column(nullable=True)
...     __mapper_args__ = {
...         "polymorphic_identity": "manager",
...         "polymorphic_load": "inline",
...     }
>>> class Engineer(Employee):
...     engineer_info: Mapped[str] = mapped_column(nullable=True)
...     __mapper_args__ = {
...         "polymorphic_identity": "engineer",
...         "polymorphic_load": "inline",
...     }

With the above mapping, the Manager and Engineer classes will have their columns included in SELECT statements against the Employee entity automatically:

>>> print(select(Employee))
SELECT employee.id, employee.name, employee.type,
employee.manager_name, employee.engineer_info
FROM employee

Inheritance Loading API

Object Name	Description
selectin_polymorphic(base_cls, classes)	Indicate an eager load should take place for all attributes specific to a subclass.
with_polymorphic(base, classes[, selectable, flat, …])	Produce an AliasedClass construct which specifies columns for descendant mappers of the given base.

function sqlalchemy.orm.with_polymorphic(base: Union[_O, Mapper[_O]], classes: Union[Literal[‘*‘], Iterable[Type[Any]]], selectable: Union[Literal[False, None], FromClause] = False, flat: bool = False, polymorphic_on: Optional[ColumnElement[Any]] = None, aliased: bool = False, innerjoin: bool = False, adapt_on_names: bool = False, _use_mapper_path: bool = False) → AliasedClass[_O]

Produce an AliasedClass construct which specifies columns for descendant mappers of the given base.

Using this method will ensure that each descendant mapper’s tables are included in the FROM clause, and will allow filter() criterion to be used against those tables. The resulting instances will also have those columns already loaded so that no “post fetch” of those columns will be required.

See also

Using with_polymorphic() - full discussion of with_polymorphic().

Parameters:
- base – Base class to be aliased.
- classes – a single class or mapper, or list of class/mappers, which inherit from the base class. Alternatively, it may also be the string '*', in which case all descending mapped classes will be added to the FROM clause.
- aliased – when True, the selectable will be aliased. For a JOIN, this means the JOIN will be SELECTed from inside of a subquery unless the with_polymorphic.flat flag is set to True, which is recommended for simpler use cases.
- flat – Boolean, will be passed through to the FromClause.alias() call so that aliases of Join objects will alias the individual tables inside the join, rather than creating a subquery. This is generally supported by all modern databases with regards to right-nested joins and generally produces more efficient queries. Setting this flag is recommended as long as the resulting SQL is functional.
- selectable –
  
  a table or subquery that will be used in place of the generated FROM clause. This argument is required if any of the desired classes use concrete table inheritance, since SQLAlchemy currently cannot generate UNIONs among tables automatically. If used, the selectable argument must represent the full set of tables and columns mapped by every mapped class. Otherwise, the unaccounted mapped columns will result in their table being appended directly to the FROM clause which will usually lead to incorrect results.
  
  When left at its default value of False, the polymorphic selectable assigned to the base mapper is used for selecting rows. However, it may also be passed as None, which will bypass the configured polymorphic selectable and instead construct an ad-hoc selectable for the target classes given; for joined table inheritance this will be a join that includes all target mappers and their subclasses.
- polymorphic_on – a column to be used as the “discriminator” column for the given selectable. If not given, the polymorphic_on attribute of the base classes’ mapper will be used, if any. This is useful for mappings that don’t have polymorphic loading behavior by default.
- innerjoin – if True, an INNER JOIN will be used. This should only be specified if querying for one specific subtype only
- adapt_on_names –
  
  Passes through the aliased.adapt_on_names parameter to the aliased object. This may be useful in situations where the given selectable is not directly related to the existing mapped selectable.
  
  New in version 1.4.33.

function sqlalchemy.orm.selectin_polymorphic(base_cls: _EntityType[Any], classes: Iterable[Type[Any]]) → _AbstractLoad

Indicate an eager load should take place for all attributes specific to a subclass.

This uses an additional SELECT with IN against all matched primary key values, and is the per-query analogue to the "selectin" setting on the mapper.polymorphic_load parameter.

New in version 1.2.

See also

Using selectin_polymorphic()

ORM Querying Guide

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