Retrieving Data

The real strength of our database is in how it allows us to retrieve datathrough queries. Relational databases are excellent for making ad-hocqueries.

Getting single records

Let’s retrieve Grandma’s record from the database. To get a single record fromthe database, use Select.get():

grandma = Person.select().where(Person.name == 'Grandma L.').get()

We can also use the equivalent shorthand Model.get():

grandma = Person.get(Person.name == 'Grandma L.')

Lists of records

Let’s list all the people in the database:

for person in Person.select():
    print(person.name)
 
# prints:
# Bob
# Grandma L.
# Herb

Let’s list all the cats and their owner’s name:

query = Pet.select().where(Pet.animal_type == 'cat')
for pet in query:
    print(pet.name, pet.owner.name)
 
# prints:
# Kitty Bob
# Mittens Jr Herb

Attention

There is a big problem with the previous query: because we are accessingpet.owner.name and we did not select this relation in our originalquery, peewee will have to perform an additional query to retrieve thepet’s owner. This behavior is referred to as N+1 and itshould generally be avoided.

For an in-depth guide to working with relationships and joins, refer to theRelationships and Joins documentation.

We can avoid the extra queries by selecting both Pet and Person, and addinga join.

query = (Pet
         .select(Pet, Person)
         .join(Person)
         .where(Pet.animal_type == 'cat'))
 
for pet in query:
    print(pet.name, pet.owner.name)
 
# prints:
# Kitty Bob
# Mittens Jr Herb

Let’s get all the pets owned by Bob:

for pet in Pet.select().join(Person).where(Person.name == 'Bob'):
    print(pet.name)
 
# prints:
# Kitty
# Fido

We can do another cool thing here to get bob’s pets. Since we already have anobject to represent Bob, we can do this instead:

for pet in Pet.select().where(Pet.owner == uncle_bob):
    print(pet.name)

Sorting

Let’s make sure these are sorted alphabetically by adding anorder_by() clause:

for pet in Pet.select().where(Pet.owner == uncle_bob).order_by(Pet.name):
    print(pet.name)
 
# prints:
# Fido
# Kitty

Let’s list all the people now, youngest to oldest:

for person in Person.select().order_by(Person.birthday.desc()):
    print(person.name, person.birthday)
 
# prints:
# Bob 1960-01-15
# Herb 1950-05-05
# Grandma L. 1935-03-01

Combining filter expressions

Peewee supports arbitrarily-nested expressions. Let’s get all the people whosebirthday was either:

• before 1940 (grandma)
• after 1959 (bob)

d1940 = date(1940, 1, 1)
d1960 = date(1960, 1, 1)
query = (Person
         .select()
         .where((Person.birthday < d1940) | (Person.birthday > d1960)))
 
for person in query:
    print(person.name, person.birthday)
 
# prints:
# Bob 1960-01-15
# Grandma L. 1935-03-01

Now let’s do the opposite. People whose birthday is between 1940 and 1960:

query = (Person
         .select()
         .where(Person.birthday.between(d1940, d1960)))
 
for person in query:
    print(person.name, person.birthday)
 
# prints:
# Herb 1950-05-05

Aggregates and Prefetch

Now let’s list all the people and how many pets they have:

for person in Person.select():
    print(person.name, person.pets.count(), 'pets')
 
# prints:
# Bob 2 pets
# Grandma L. 0 pets
# Herb 1 pets

Once again we’ve run into a classic example of N+1 querybehavior. In this case, we’re executing an additional query for everyPerson returned by the original SELECT! We can avoid this by performinga JOIN and using a SQL function to aggregate the results.

query = (Person
         .select(Person, fn.COUNT(Pet.id).alias('pet_count'))
         .join(Pet, JOIN.LEFT_OUTER)  # include people without pets.
         .group_by(Person)
         .order_by(Person.name))
 
for person in query:
    # "pet_count" becomes an attribute on the returned model instances.
    print(person.name, person.pet_count, 'pets')
 
# prints:
# Bob 2 pets
# Grandma L. 0 pets
# Herb 1 pets

Note

Peewee provides a magical helper fn(), which can be used to callany SQL function. In the above example, fn.COUNT(Pet.id).alias('pet_count')would be translated into COUNT(pet.id) AS pet_count.

Now let’s list all the people and the names of all their pets. As you may haveguessed, this could easily turn into another N+1 situation ifwe’re not careful.

Before diving into the code, consider how this example is different from theearlier example where we listed all the pets and their owner’s name. A pet canonly have one owner, so when we performed the join from Pet to Person,there was always going to be a single match. The situation is different when weare joining from Person to Pet because a person may have zero pets orthey may have several pets. Because we’re using a relational databases, if wewere to do a join from Person to Pet then every person with multiplepets would be repeated, once for each pet.

It would look like this:

query = (Person
         .select(Person, Pet)
         .join(Pet, JOIN.LEFT_OUTER)
         .order_by(Person.name, Pet.name))
for person in query:
    # We need to check if they have a pet instance attached, since not all
    # people have pets.
    if hasattr(person, 'pet'):
        print(person.name, person.pet.name)
    else:
        print(person.name, 'no pets')
 
# prints:
# Bob Fido
# Bob Kitty
# Grandma L. no pets
# Herb Mittens Jr

Usually this type of duplication is undesirable. To accommodate the more common(and intuitive) workflow of listing a person and attaching a list of thatperson’s pets, we can use a special method calledprefetch():

query = Person.select().order_by(Person.name).prefetch(Pet)
for person in query:
    print(person.name)
    for pet in person.pets:
        print('  *', pet.name)
 
# prints:
# Bob
#   * Kitty
#   * Fido
# Grandma L.
# Herb
#   * Mittens Jr

SQL Functions

One last query. This will use a SQL function to find all people whose namesstart with either an upper or lower-case G:

expression = fn.Lower(fn.Substr(Person.name, 1, 1)) == 'g'
for person in Person.select().where(expression):
    print(person.name)
 
# prints:
# Grandma L.

This is just the basics! You can make your queries as complex as you like.Check the documentation on Querying for more info.