SQLAlchemy: Mastering One Query to Join Several Tables

When working with relational databases, it’s common to need information from multiple tables. In this blog post, we will explore how to join several tables by one query using SQLAlchemy in a professional, conversational, and human-like tone. So, let’s dive in!

Understanding the Scenario

Let’s say you have the following 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"))

You want to fetch a table with the following structure for a given user email:

email | name | document_name | document_readAllowed | document_writeAllowed

Joining Tables in One Query

To achieve this, you can use the following query:

q = Session.query(
         User, Document, DocumentsPermissions,
    ).filter(
         User.email == Document.author,
    ).filter(
         Document.name == DocumentsPermissions.document,
    ).filter(
        User.email == 'user@email.com',
    ).all()

Breaking Down the Query

• The Session.query() method is used to define the columns from the three tables you want to fetch.
• The filter() method is applied three times to define the conditions for joining the tables and fetching the desired user’s data.
• Finally, the all() method is called to execute the query and fetch the results.

Additional Tips

Define Relationships and Primary Keys

It’s good practice to define primary keys for all tables and set up relationships between them. This allows for better ORM querying and a more organized database structure. In the given example, consider adding integer primary keys and defining relationships between the classes.

Use the Join Method

Instead of using multiple filter() methods, consider using the join() method for a cleaner and more efficient query. This way, you can explicitly specify the relationship between the tables, making the code more readable and maintainable.

Print the Query for Debugging

If you want to see the raw SQL query generated by SQLAlchemy, simply remove the all() method and print the query. This can be useful for debugging and understanding the underlying SQL statements.

Conclusion

Joining multiple tables in one query using SQLAlchemy is straightforward once you understand the process. By defining primary keys, relationships, and using the appropriate query methods, you can easily fetch the desired data from multiple tables. Remember to keep your code clean and organized, and don’t hesitate to use built-in methods and tools for debugging and optimization.