sqlalchemy将两个表连接在一起

Question

I am wanting to map a class object to a table that is a join between two tables, and all the columns from one table and only one column from the joined table being selected (mapped).

join_table = join(table1, table2, tabl1.c.description==table2.c.description)
model_table_join= select([table1, table2.c.description]).select_from(join_table).alias()

Am I doing this right?

Answer 1

If all you want to do is pull in one extra column from a JOIN, I'd not muck about with an arbitrary select mapping. As the documentation points out :

The practice of mapping to arbitrary SELECT statements, especially complex ones as above, is almost never needed; it necessarily tends to produce complex queries which are often less efficient than that which would be produced by direct query construction. The practice is to some degree based on the very early history of SQLAlchemy where the mapper() construct was meant to represent the primary querying interface; in modern usage, the Query object can be used to construct virtually any SELECT statement, including complex composites, and should be favored over the “map-to-selectable” approach.

You'd just either select that extra column in your application:

session.query(Table1Model, Table2Model.description).join(Table2Model)

or you can register a relationship on the Table1Model and an association property that always pulls in the extra column:

class Table1Model(Base):
    # ...

    _table2 = relationship('Table2Model', lazy='join')
    description = association_proxy('_table2', 'description')

The association property manages the Table2Model.description column of the joined row as you interact with it on Table1Model instances.

That said, if you must stick with a join() query as the base, then you could just exclude the extra, duplicated columns from the join, with a exclude_properties mapper argument :

join_table = join(table1, table2, table1.c.description == table2.c.description)

class JoinedTableModel(Base):
    __table__ = join_table
    __mapper_args__ = {
        'exclude_properties' : [table1.c.description]
    }

The new model then uses all the columns from the join to create attributes with the same names, except for those listed in `exclude_properties.

Or you can keep using duplicated column names in the model simply by giving them a new name:

join_table = join(table1, table2, table1.c.description == table2.c.description)

class JoinedTableModel(Base):
    __table__ = join_table

    table1_description = table1.c.description

You can rename any column from the join this way, at which point they will no longer conflict with other columns with the same base name from the other table.