How to Join Multiple Tables in a Single SQLAlchemy Query?

Joins for Multiple Tables in a Single SQLAlchemy Query

In SQLAlchemy, joining tables allows you to combine data from multiple related tables into a single result set. This can be useful when you need to query data across tables that have relationships, such as a user with multiple documents.

To join multiple tables using SQLAlchemy, you can use the join() method. This method takes two arguments: the first argument is the target table, and the second argument is the condition that specifies how the tables are joined.

For example, consider the following table structure:

class User(Base):
    __tablename__ = 'users'
    email = Column(String, primary_key=True)
    name = Column(String)

class Document(Base):
    __tablename__ = "documents"
    name = Column(String, primary_key=True)
    author = Column(String, ForeignKey("users.email"))

class DocumentsPermissions(Base):
    __tablename__ = "documents_permissions"
    readAllowed = Column(Boolean)
    writeAllowed = Column(Boolean)

    document = Column(String, ForeignKey("documents.name"))

To query all the documents and permissions for a user with email "[email protected]", you can use the following query:

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

This query performs a join between the User, Document, and DocumentsPermissions tables, filtering the results to only include documents that are authored by the user with email "someemail". The result is a list of tuples containing the user, document, and permissions data.

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