Updating Existing Records—Another Use for WHERE（更新已有的记录——WHERE 再战江湖）

Now that you’ve got nice generalized select and where functions, you’re in a good position to write the next feature that every database needs—a way to update particular records. In SQL the update command is used to update a set of records matching a particular where clause. That seems like a good model, especially since you’ve already got a where-clause generator. In fact, the update function is mostly just the application of a few ideas you’ve already seen: using a passed-in selector function to choose the records to update and using keyword arguments to specify the values to change. The main new bit is the use of a function MAPCAR that maps over a list, *db* in this case, and returns a new list containing the results of calling a function on each item in the original list.

有了完美通用的 select 和 where 函数，是时候开始编写下一个所有数据库都需要的特性——更新特定记录的方法了。在 SQL 中，update 命令被用于更新一组匹配特定 where 子句的记录。这听起来像是个很好的模型，尤其是当已经有了一个 where 子句生成器时。事实上，update 函数只是你已经见过的一些思路的再次应用：使用一个通过参数传递的选择器函数来选取需要更新的记录，再使用关键字形参来指定需要改变的值。这里主要出现的新内容是对 MAPCAR 函数的使用，其映射在一个列表上（这里是 *db*），然后返回一个新的列表，其中含有在原来列表的每个 元素上调用一个函数所得到的结果。

  1. (defun update (selector-fn &key title artist rating (ripped nil ripped-p))
  2.   (setf *db*
  3.         (mapcar
  4.          #'(lambda (row)
  5.              (when (funcall selector-fn row)
  6.                (if title    (setf (getf row :title) title))
  7.                (if artist   (setf (getf row :artist) artist))
  8.                (if rating   (setf (getf row :rating) rating))
  9.                (if ripped-p (setf (getf row :ripped) ripped)))
  10.              row) *db*)))

One other new bit here is the use of SETF on a complex form such as (getf row :title). I’ll discuss SETF in greater detail in Chapter 6, but for now you just need to know that it’s a general assignment operator that can be used to assign lots of “places” other than just variables. (It’s a coincidence that SETF and GETF have such similar names—they don’t have any special relationship.) For now it’s enough to know that after (setf (getf row :title) title), the plist referenced by row will have the value of the variable title following the property name :title. With this update function if you decide that you really dig the Dixie Chicks and that all their albums should go to 11, you can evaluate the following form:

这里的另一个新内容是 SETF 用在了诸如 (getf row :title) 这样的复杂形式上。第 6 章将细致地讨论 SETF，目前只需知道它是一个通用的赋值操作符，可被用于对各种 “位置” 而不只是对变量进行赋值即可。（SETF 和 GETF 具有相似的名字，但这纯属巧合，两者之间并没有特别的关系。）眼下知道执行 (setf (getf row :title) title) 以后的结果就可以了：由 row 所引用的 plist 将具有紧跟着属性名 :title 后面的那项变量 title 的值。有了这个 update 函数，如果你觉得自己真的很喜欢 Dixie Chicks，并且他们的所有专辑的评级应该升到 11，那么可以对下列形式求值：

  1. CL-USER> (update (where :artist "Dixie Chicks") :rating 11)
  2. NIL

And it is so.

这样就可以了。

  1. CL-USER> (select (where :artist "Dixie Chicks"))
  2. ((:TITLE "Home" :ARTIST "Dixie Chicks" :RATING 11 :RIPPED T)
  3.  (:TITLE "Fly" :ARTIST "Dixie Chicks" :RATING 11 :RIPPED T))

You can even more easily add a function to delete rows from the database.

甚至可以更容易地添加一个函数来从数据库里删除行。

  1. (defun delete-rows (selector-fn)
  2.   (setf *db* (remove-if selector-fn *db*)))

The function REMOVE-IF is the complement of REMOVE-IF-NOT; it returns a list with all the elements that do match the predicate removed. Like REMOVE-IF-NOT, it doesn’t actually affect the list it’s passed but by saving the result back into *db*, delete-rows actually changes the contents of the database.

函数 REMOVE-IF 的功能跟 REMOVE-IF-NOT 正好相反，在它所返回的列表中，所有确实匹配谓词的元素都被删掉的。和 REMOVE-IF-NOT 一样，它不会实际地影响传入的那个列表，但是通过将结果重新保存到 *db* 中，delete-rows 事实上改变了数据库的内容。