How to Efficiently Join Multiple Tables in a Single SQLAlchemy Query?

SQLAlchemy: Joining Multiple Tables in a Single Query

In SQLAlchemy, joining multiple tables is essential for retrieving complex data. Consider the following scenario where you have three mapped classes: User, Document, and DocumentPermissions. The objective is to retrieve a customized table that combines these tables based on specific conditions, similar to the following:

email | name | document_name | document_readAllowed | document_writeAllowed

Initial Approach and its Limitations

An initial attempt to join the tables using the following query may not yield the desired result:

result = session.query(User, Document, DocumentPermission).filter_by(email = "[email protected]").all()

Improved Query using Intermediated Tables

To effectively join the tables, it is necessary to utilize intermediate tables that connect the primary key of one table to the foreign key of another. In this case, Document connects to User through the author column, and DocumentsPermissions connects to Document through the document column.

The improved query leverages the intermediate tables to establish relationships between the tables:

q = Session.query(
         User, Document, DocumentPermissions,
    ).filter(
         User.email == Document.author,
    ).filter(
         Document.name == DocumentPermissions.document,
    ).filter(
        User.email == 'someemail',
    ).all()

By following these steps, you can efficiently join multiple tables in a single SQLAlchemy query and retrieve the desired data.

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