How to Perform a Single SQLAlchemy Query Joining Multiple Tables?

Joining Multiple Tables in SQLAlchemy with a Single Query

To execute a comprehensive join operation involving multiple tables in SQLAlchemy, it's essential to follow a specific approach. Consider the provided SQLAlchemy mapped classes:

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 retrieve a result in the desired format for a specific user, follow these steps:

1. Start by querying the necessary tables using the query function:

q = Session.query(
    User, Document, DocumentPermissions
)

2. Establish relationships between the tables using the filter method:

q.filter(
    User.email == Document.author
)

3. Include additional relationships as needed:

q.filter(
    Document.name == DocumentPermissions.document
)

4. Specify the filtering condition based on the desired user email:

q.filter(
    User.email == 'someemail'
)

5. Finally, execute the query using the all method to retrieve the results.

By following this approach, it becomes possible to efficiently join multiple tables using a single SQLAlchemy query.

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