How Can I Efficiently Join Multiple Tables (Users, Documents, Permissions) in SQLAlchemy?

Joining Multiple Tables in SQLAlchemy

In SQLAlchemy, joining tables enables the combination of data from multiple related entities. To illustrate this, consider the following example:

Suppose you have the following mapped classes representing users, documents, and document permissions:

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 table that includes information about users, documents, and document permissions for a specific user, 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:

1. Joins the User table with the Document table based on the relationship between the email and author columns.
2. Joins the Document table with the DocumentsPermissions table based on the relationship between the name column in both tables.
3. Filters the results to include only rows where the User.email equals a specified value ('someemail') in this example.

By executing this query, you can efficiently retrieve all relevant information from the three related tables in a single request, providing a comprehensive view of data pertaining to the specified user.

The above is the detailed content of How Can I Efficiently Join Multiple Tables (Users, Documents, Permissions) in SQLAlchemy?. For more information, please follow other related articles on the PHP Chinese website!