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 == 'email@example.com', ).all()
Breaking Down the Query
Session.query()method is used to define the columns from the three tables you want to fetch.
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.
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.
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.