Relations on composite keys using sqlalchemy

Relations on composite keys using sqlalchemy

In SQLAlchemy, you can define relationships between tables that have composite primary keys (composite keys are primary keys consisting of multiple columns). To establish relationships, you'll typically use the relationship() function along with ForeignKey objects. Here's an example of how to define relationships on tables with composite keys:

Suppose you have two tables, Author and Book, and you want to establish a one-to-many relationship where each author can have multiple books. Both tables have composite primary keys.

from sqlalchemy import create_engine, Column, Integer, String, ForeignKey from sqlalchemy.orm import relationship, sessionmaker from sqlalchemy.ext.declarative import declarative_base Base = declarative_base() class Author(Base): __tablename__ = 'authors' author_id = Column(Integer, primary_key=True) author_name = Column(String) # Define the relationship with Book books = relationship('Book', back_populates='author') class Book(Base): __tablename__ = 'books' book_id = Column(Integer, primary_key=True) book_title = Column(String) # Composite foreign key referencing Author author_id = Column(Integer, ForeignKey('authors.author_id'), primary_key=True) # Define the relationship with Author author = relationship('Author', back_populates='books') # Create a SQLite in-memory database for this example engine = create_engine('sqlite:///:memory:') # Create the tables Base.metadata.create_all(engine) # Create a session Session = sessionmaker(bind=engine) session = Session() # Adding data to the database author1 = Author(author_name='Author 1') book1 = Book(book_title='Book 1', author=author1) book2 = Book(book_title='Book 2', author=author1) session.add_all([author1, book1, book2]) session.commit() # Query the data with relationships author = session.query(Author).filter_by(author_name='Author 1').first() print("Author:", author.author_name) print("Books written by the author:") for book in author.books: print(book.book_title)

In this example:

  • Author and Book are two tables with composite primary keys.
  • The Author table has a one-to-many relationship with the Book table using the relationship() function.
  • The ForeignKey object in the Book table is used to establish the foreign key relationship with the Author table's composite primary key.
  • You can query the data with relationships, as shown in the example.

This demonstrates how to define relationships on tables with composite primary keys using SQLAlchemy.

Examples

  1. How to define composite primary keys in SQLAlchemy?

    • This query discusses creating tables with composite primary keys in SQLAlchemy.
    from sqlalchemy import create_engine, Column, Integer, String, ForeignKey, PrimaryKeyConstraint from sqlalchemy.ext.declarative import declarative_base Base = declarative_base() class CompositeKeyExample(Base): __tablename__ = 'composite_key_example' column1 = Column(Integer, primary_key=True) column2 = Column(Integer, primary_key=True) value = Column(String(50)) engine = create_engine("sqlite:///:memory:") Base.metadata.create_all(engine) # Creates the table with composite keys

  2. How to create a foreign key relation using composite keys in SQLAlchemy?

    • This query shows how to create a relationship with foreign keys that reference composite primary keys.
    from sqlalchemy import create_engine, Column, Integer, String, ForeignKey from sqlalchemy.ext.declarative import declarative_base from sqlalchemy.orm import relationship, sessionmaker Base = declarative_base() # Table with composite primary key class Parent(Base): __tablename__ = 'parent' id1 = Column(Integer, primary_key=True) id2 = Column(Integer, primary_key=True) name = Column(String(50)) # Table referencing composite primary key class Child(Base): __tablename__ = 'child' id1 = Column(Integer, primary_key=True) id2 = Column(Integer, primary_key=True) parent_id1 = Column(Integer, ForeignKey("parent.id1")) parent_id2 = Column(Integer, ForeignKey("parent.id2")) parent = relationship("Parent", primaryjoin="and_(Child.parent_id1 == Parent.id1, Child.parent_id2 == Parent.id2)") engine = create_engine("sqlite:///:memory:") Base.metadata.create_all(engine) # Create a session and add sample data Session = sessionmaker(bind=engine) session = Session() parent = Parent(id1=1, id2=2, name="Parent Example") child = Child(id1=3, id2=4, parent_id1=1, parent_id2=2) session.add(parent) session.add(child) session.commit() # Save changes

  3. How to establish relationships between tables with composite keys in SQLAlchemy?

    • This query explains how to establish relationships between tables with composite keys.
    from sqlalchemy import create_engine, Column, Integer, String, ForeignKey, PrimaryKeyConstraint from sqlalchemy.ext.declarative import declarative_base from sqlalchemy.orm import relationship, sessionmaker Base = declarative_base() class Category(Base): __tablename__ = 'category' cat_id1 = Column(Integer, primary_key=True) cat_id2 = Column(Integer, primary_key=True) name = Column(String(50)) class Product(Base): __tablename__ = 'product' prod_id = Column(Integer, primary_key=True) cat_id1 = Column(Integer, ForeignKey("category.cat_id1")) cat_id2 = Column(Integer, ForeignKey("category.cat_id2")) name = Column(String(50)) category = relationship("Category", primaryjoin="and_(Product.cat_id1 == Category.cat_id1, Product.cat_id2 == Category.cat_id2)") engine = create_engine("sqlite:///:memory:") Base.metadata.create_all(engine) # Create a session and add sample data Session = sessionmaker(bind=engine) session = Session() category = Category(cat_id1=1, cat_id2=2, name="Category 1") product = Product(prod_id=10, cat_id1=1, cat_id2=2, name="Product A") session.add(category) session.add(product) session.commit()

  4. How to query tables with composite key relationships in SQLAlchemy?

    • This query demonstrates querying data in tables with composite key relationships.
    from sqlalchemy import create_engine, Column, Integer, String, ForeignKey from sqlalchemy.ext.declarative import declarative_base from sqlalchemy.orm import relationship, sessionmaker Base = declarative_base() class Author(Base): __tablename__ = 'author' id1 = Column(Integer, primary_key=True) id2 = Column(Integer, primary_key=True) name = Column(String(50)) class Book(Base): __tablename__ = 'book' id1 = Column(Integer, primary_key=True) id2 = Column(Integer, ForeignKey("author.id1")) id3 = Column(Integer, ForeignKey("author.id2")) title = Column(String(100)) author = relationship("Author", primaryjoin="and_(Book.id2 == Author.id1, Book.id3 == Author.id2)") engine = create_engine("sqlite:///:memory:") Base.metadata.create_all(engine) # Create a session Session = sessionmaker(bind=engine) session = Session() # Add data to the session author = Author(id1=1, id2=2, name="John Doe") book = Book(id1=3, id2=1, id3=2, title="Book Title") session.add(author) session.add(book) session.commit() # Query the book with related author using composite key queried_book = session.query(Book).filter(Book.id2 == 1, Book.id3 == 2).first() print(queried_book.title) # Output: "Book Title" print(queried_book.author.name) # Output: "John Doe"

  5. How to define unique constraints with composite keys in SQLAlchemy?

    • This query demonstrates setting unique constraints on composite keys to ensure unique combinations.
    from sqlalchemy import create_engine, Column, Integer, String, UniqueConstraint from sqlalchemy.ext.declarative import declarative_base from sqlalchemy.orm import sessionmaker Base = declarative_base() class User(Base): __tablename__ = 'user' id = Column(Integer, primary_key=True) first_name = Column(String(50)) last_name = Column(String(50)) email = Column(String(100)) __table_args__ = (UniqueConstraint("first_name", "last_name", name="unique_user_name"),) engine = create_engine("sqlite:///:memory:") Base.metadata.create_all(engine) # Create a session Session = sessionmaker(bind=engine) session = Session() # Adding duplicate users should fail due to unique constraint user1 = User(first_name="John", last_name="Doe", email="john.doe@example.com") session.add(user1) session.commit() user2 = User(first_name="John", last_name="Doe", email="john.different@example.com") try: session.add(user2) session.commit() except Exception as e: session.rollback() # Rollback the session on error print("Error: Unique constraint violation") # Expected error due to unique constraint

  6. How to use composite keys for indexing in SQLAlchemy?

    • This query demonstrates using composite keys for creating indexes to optimize query performance.
    from sqlalchemy import create_engine, Column, Integer, String, Index from sqlalchemy.ext.declarative import declarative_base from sqlalchemy.orm import sessionmaker Base = declarative_base() class Order(Base): __tablename__ = 'order' order_id = Column(Integer, primary_key=True) customer_id = Column(Integer, nullable=False) product_id = Column(Integer, nullable=False) quantity = Column(Integer, nullable=False) # Create index on customer_id and product_id for query performance __table_args__ = (Index("ix_customer_product", "customer_id", "product_id"),) engine = create_engine("sqlite:///:memory:") Base.metadata.create_all(engine) # Create a session Session = sessionmaker(bind=engine) session = Session() # Adding some data for querying and checking index performance order1 = Order(order_id=1, customer_id=100, product_id=200, quantity=10) order2 = Order(order_id=2, customer_id=100, product_id=201, quantity=5) session.add(order1) session.add(order2) session.commit() # Query to find orders by customer_id and product_id queried_order = session.query(Order).filter_by(customer_id=100, product_id=200).first() print(queried_order.quantity) # Output: 10

  7. How to use composite keys for ForeignKey relationships in SQLAlchemy?

    • This query shows how to use composite keys as ForeignKey targets in related tables.
    from sqlalchemy import create_engine, Column, Integer, ForeignKey, String from sqlalchemy.ext.declarative import declarative_base from sqlalchemy.orm import sessionmaker, relationship Base = declarative_base() class Department(Base): __tablename__ = 'department' dept_id1 = Column(Integer, primary_key=True) dept_id2 = Column(Integer, primary_key=True) name = Column(String(50)) class Employee(Base): __tablename__ = 'employee' emp_id = Column(Integer, primary_key=True) dept_id1 = Column(Integer, ForeignKey("department.dept_id1")) dept_id2 = Column(Integer, ForeignKey("department.dept_id2")) name = Column(String(50)) department = relationship("Department", primaryjoin="and_(Employee.dept_id1 == Department.dept_id1, Employee.dept_id2 == Department.dept_id2)") engine = create_engine("sqlite:///:memory:") Base.metadata.create_all(engine) # Create a session and add some data Session = sessionmaker(bind=engine) session = Session() dept = Department(dept_id1=1, dept_id2=2, name="HR") employee = Employee(emp_id=100, name="Alice", dept_id1=1, dept_id2=2) session.add(dept) session.add(employee) session.commit() # Save changes

  8. How to use composite keys with primary key and foreign key relationships in SQLAlchemy?

    • This query demonstrates using composite keys in tables that contain both primary key and foreign key relationships.
    from sqlalchemy import create_engine, Column, Integer, String, ForeignKey from sqlalchemy.ext.declarative import declarative_base from sqlalchemy.orm import sessionmaker, relationship Base = declarative_base() class School(Base): __tablename__ = 'school' school_id1 = Column(Integer, primary_key=True) school_id2 = Column(Integer, primary_key=True) name = Column(String(50)) class Student(Base): __tablename__ = 'student' student_id = Column(Integer, primary_key=True) school_id1 = Column(Integer, ForeignKey("school.school_id1")) school_id2 = Column(Integer, ForeignKey("school.school_id2")) name = Column(String(50)) school = relationship("School", primaryjoin="and_(Student.school_id1 == School.school_id1, Student.school_id2 == School.school_id2)") engine = create_engine("sqlite:///:memory:") Base.metadata create_all(engine) # Create a session and add sample data Session = sessionmaker(bind=engine) session = Session() school = School(school_id1=1, school_id2=2, name="ABC School") student = Student(student_id=10, school_id1=1, school_id2=2, name="John") session.add(school) session.add(student) session.commit() # Save changes

  9. How to define composite keys with auto-increment in SQLAlchemy?

    • This query explores whether composite keys can be used with auto-incrementing primary keys in SQLAlchemy.
    from sqlalchemy import create_engine, Column, Integer, String from sqlalchemy.ext.declarative import declarative_base from sqlalchemy.orm import sessionmaker Base = declarative_base() class Product(Base): __tablename__ = 'product' prod_id = Column(Integer, primary_key=True, autoincrement=True) name = Column(String(50)) category = Column(String(50)) engine = create_engine("sqlite:///:memory:") Base.metadata.create_all(engine) # Create a session and add some data Session = sessionmaker(bind=engine) session = Session() product1 = Product(name="Product 1", category="Category A") product2 = Product(name="Product 2", category="Category B") session.add(product1) session.add(product2) session.commit() # Check auto-incrementing IDs with composite key print(product1.prod_id) # Output: 1 print(product2.prod_id) # Output: 2

  10. How to use composite keys with relationships and join tables in SQLAlchemy?

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