Mapping a Subset of Table Columns

Mapping a Subset of Table Columns

(SQLAlchemy ORM > ORM Mapped Class Configuration > Mapping Columns and Expressions > Mapping Table Columns)

Sometimes, a Table object was made available using the reflection process descirbed at Reflecting Database Objects to load the table's structure from the database. For such a table that has lots of columns that don't need to be referenced in the application.

For example:

class User(Base):
	__table__ = user_table
    __mapper_args__ = {
    	'include_properties': ['user_id', 'user_name']
 	}

...will map the User class to the user_table table, only including the user_id, and user_name columns - the rest are not referenced.

Similarly:

class Address(Base):
	__table__ = address_table
    __mapper_args__ = {
    	'exclude_properties': ['street', 'city', 'state', 'zip']
    }

...will map the Address class to the address_table table, including all columns present excpet street, city, state, and zip.

When this mapping is used, the columns that are not included will not be referenced by in any SELECT statements emitted by Query, nor will there be any mapped attribute on the mapped class which represents the column; assigning an attribute of that name will have no effect beyond that of a normal Python attribute assignment.

In some cases, multiple columns may have the same name, such as when mapping to a join of two or more tables that share some column name. include_properties and exclude_properties can also accommodate Column objects to more accurately describe which columns should be included or excluded:

class UserAddress(Base):
	__table__ = user_table.join(addresses_table)
    __mapper_args__ = {
    	'exclude_properties': [address_table.c.id],
        'primary_key': [user_table.c.id]
    }

Note
insert and update defaults configured on individual Column objecs, i.e. those described at Column INSERT/UPDATE Defaults including those configured by the Column.default, Column.onupdate, Column.server_default and Column.server_onupdate parameters, will continue to function normally even if those Column objects are not mapped. This is because in the case of Column.default and Column.onupdate, the Column object is till present on the underlying Table, thus allowing the default functions to take place when the ORM emits an INSERT or UPDATE, and in the case of Column.server_default and Column.server_onupdate, the relational database itself emits these defaults as a server side behaivor.

__flow__ comments

• 무슨 말인지 잘 모르겠다. TWIL에서 다시 읽어보자. (2022.07.11.)