Upsert in SQLAlchemy with PostgreSQL: Efficiency for Supported Databases

1. Query first, create if not found: This approach involves two steps:

   • Query: You write a query to check if the object exists in the database based on unique identifiers like an ID or a combination of fields.
   • Create: If the query returns nothing (meaning the object doesn't exist), you create a new object and add it to the database session.

2. Upsert (insert or update): This functionality isn't built-in to SQLAlchemy, but some databases like PostgreSQL offer an "upsert" feature. Here's the idea:

   • You create an insert statement specifying the data for the new object.
   • You define a conflict resolution clause. This clause tells the database what to do if a record with the same unique identifier already exists (e.g., do nothing, update existing record).

Here's a comparison:

• Query first, create if not found: This is more portable as it works with any database supported by SQLAlchemy. However, it requires two database calls (query and insert).
• Upsert: This can be more efficient with databases that support it as it's a single database operation. However, it's not universally available.

from sqlalchemy import create_engine, Column, Integer, String, and_

# Define your database connection
engine = create_engine('your_database_uri')

# Define your model class
class User(Base):
  __tablename__ = 'users'
  id = Column(Integer, primary_key=True)
  name = Column(String)

# Function to check if user exists and create if not
def create_user_if_not_exists(session, username):
  # Query for existing user with specific username
  existing_user = session.query(User).filter(User.name == username).first()

  if not existing_user:
    # Create a new user object if not found
    new_user = User(name=username)
    session.add(new_user)
    session.commit()

# Example usage
session = Session(engine)
create_user_if_not_exists(session, "Alice")
session.close()

Upsert using PostgreSQL (example):

from sqlalchemy import create_engine, Column, Integer, String, and_, insert

# Define your database connection (PostgreSQL assumed here)
engine = create_engine('postgresql://user:password@host/database')

# Define your model class
class User(Base):
  __tablename__ = 'users'
  id = Column(Integer, primary_key=True)
  name = Column(String, unique=True)  # Unique constraint for upsert

# Function to upsert a user (insert or update)
def upsert_user(session, username):
  # Create insert statement with conflict resolution
  stmt = insert(User).values(name=username) \
      .on_conflict_do_nothing()  # Update existing if conflict (unique name)

  session.execute(stmt)
  session.commit()

# Example usage
session = Session(engine)
upsert_user(session, "Bob")
session.close()

SQLAlchemy's session.merge method allows merging objects into the current session. We can leverage this to check for existing objects and create new ones if needed. Here's how:

from sqlalchemy import create_engine, Column, Integer, String, and_

# Define your database connection
engine = create_engine('your_database_uri')

# Define your model class
class User(Base):
  __tablename__ = 'users'
  id = Column(Integer, primary_key=True)
  name = Column(String)

# Function to create or update user (using merge)
def create_or_update_user(session, username):
  # Create a new user object
  user = User(name=username)

  # Merge the object into the session, checking for conflicts
  merged_user = session.merge(user)

  # Detach the merged object if it's new (created)
  if merged_user is not user:
    session.add(merged_user)

  session.commit()

# Example usage
session = Session(engine)
create_or_update_user(session, "Charlie")
session.close()

This approach uses merge to handle both creating a new object and updating an existing one. We check if the merged_user is the same object we created (user). If they are different, it means a conflict occurred (existing object), and we detach the merged object (which has the correct database ID) and add it to the session.

Custom SQL with execute:

This method involves writing custom SQL to achieve the upsert functionality. Here's a basic example:

from sqlalchemy import create_engine, Column, Integer, String, and_, text

# Define your database connection
engine = create_engine('your_database_uri')

# Define your model class
class User(Base):
  __tablename__ = 'users'
  id = Column(Integer, primary_key=True)
  name = Column(String, unique=True)  # Unique constraint for insert

# Function to upsert a user (custom SQL)
def upsert_user_sql(session, username):
  # Define upsert SQL statement (assuming PostgreSQL)
  stmt = text("INSERT INTO users (name) VALUES (:name) ON CONFLICT (name) DO NOTHING")
  session.execute(stmt, {'name': username})
  session.commit()

# Example usage
session = Session(engine)
upsert_user_sql(session, "David")
session.close()