ActiveMapper Crash Course

ActiveMapper is a SQLAlchemy extension developed by Jonathan LaCour which provides a declarative interface to the SQLAlchemy mapper. This page briefly covers all the extension’s features but doesn’t explain them in depth.

Note that ActiveMapper has been discontinued in favor of the Elixir declarative layer, and recent versions of SQLAlchemy come with built-in declarative support.

We’ll start off with an example:

class Demo(ActiveMapper):
    class mapping:
        ## Showing options here with default values
        #__table__ = "demo"      # class_name.lower()
        #__autoload__ = False"
        demo_key = column(Unicode(16), primary_key=True)
        name = column(Unicode(20))
        active_count = column(Integer,colname="ACTIVECOUNT")

        # a foreign key reference, note the string is the db column
        group = column(Integer, foreign_key="group.group_id", index=True)

        ## This also works
        #group = column(Integer, foreign_key= ForeignKey("group.group_id"),
                        index= True)

        #one_to_one and one_to_many only need the target AM class
        demo_link = one_to_one("DemoLink", backref="linked_demo")

        # A many_to_many mapping requires the target ActiveMapper class and
        #  an intermediate table.
        user = many_to_many("User",demo_user)

Similar to SQLObject, you declare a class that inherits from ActiveMapper. Unlike SQLObject, you must declare an internal class called mapping, which will contain all your column definitions. column() itself is a straight wrapper around the standard SQLAlchemy Column and takes the same arguments.

The relationship columns, one_to_one and many_to_many are a bit more restrictive, they take (only) the following arguments:

one_to_one & one_to_many

classname

The linked table, either as a reference or as a string containing the class name. Required.

colname

The database name of the constraint, default is None

backref

Same as standard SA, if present it creates a reference back to the current object with the same name as the passed string, default is None

private

If True, cascades deletion, default False

lazy

Delays loading the referenced object untill it is accessed, default True

order_by

Same as standard SA

many_to_many

classname

The linked table, either as a reference or as a string containing the class name. Required.

secondary

A reference to the intermediate table. Required.

backref

Same as standard SA, if present it creates a reference back to the current object with the same name as the passed string, default is None

lazy

Delays loading the referenced object untill it is accessed, default True

order_by

Same as standard SA

Most of this is demonstrated by the Identity classes created during quickstart.