Ensuring Order When Adding Related Items in SQLAlchemy

• Order not guaranteed: SQLAlchemy doesn't inherently track the order you add things. The database might store them differently.
• Many-to-one/Many-to-many relationships: These connect multiple items in your program. Imagine a blog post with many tags.

If you need the order to be maintained, there are two approaches:

1. Order column: Add a separate column in your model to specify the order for each related item. This requires extra code but ensures control.
2. OrderingList extension: SQLAlchemy extensions like "OrderingList" can manage the order within your program and handle database persistence accordingly.

from sqlalchemy import Column, Integer, ForeignKey
from sqlalchemy.orm import relationship

class Tag(Base):
  __tablename__ = "tags"
  id = Column(Integer, primary_key=True)
  name = Column(String)

class Post(Base):
  __tablename__ = "posts"
  id = Column(Integer, primary_key=True)
  title = Column(String)
  
  # Order column for tags
  tags = relationship(Tag, secondary="post_tags", order_by="post_tags.order")
  
class PostTag(Base):
  __tablename__ = "post_tags"
  post_id = Column(Integer, ForeignKey(Post.id), primary_key=True)
  tag_id = Column(Integer, ForeignKey(Tag.id), primary_key=True)
  order = Column(Integer)  # This column stores the order of the tag for this post

In this example:

• When accessing post.tags, they will be ordered based on the order column.
• A separate PostTag table links them with an order column to specify the order for each tag associated with a post.
• We define a Post and Tag model.

Using SQLAlchemy OrderingList extension (external library):

(Note: This requires installing the sqlalchemy-orderinglist extension)

from sqlalchemy import Column, Integer, ForeignKey
from sqlalchemy.orm import relationship
from sqlalchemy_orderinglist import OrderingList

class Tag(Base):
  __tablename__ = "tags"
  id = Column(Integer, primary_key=True)
  name = Column(String)

class Post(Base):
  __tablename__ = "posts"
  id = Column(Integer, primary_key=True)
  title = Column(String)
  
  tags = relationship(Tag, order_by="position", collection_class=OrderingList)

Here:

• This extension manages the order within the program and persists it to the database.
• We define a custom collection class OrderingList for the tags relationship in the Post model.
• We import OrderingList from the sqlalchemy-orderinglist extension.

Instead of relying on the database to maintain order, you can implement the logic within your program. This involves writing custom queries that sort the related items based on a desired criteria.

For example, you could sort the related items by a specific attribute of the related model when retrieving them from the database.

Positional arguments when adding related items:

While SQLAlchemy doesn't guarantee order by default, you can leverage the order you add items in some cases. This works best for scenarios where you have a limited number of related items and adding them in the desired order is sufficient.

Leveraging database specific features (limited applicability):

Some databases offer features like insertion order guarantees or user-defined sequences. However, this approach is highly database specific and might not be portable across different database backends. It's recommended to rely on SQLAlchemy features or extensions for broader compatibility.

Choosing the right approach:

The best method depends on your specific needs:

• Persistent order with complex scenarios: For scenarios where order needs to be persisted across database operations and involve a larger number of related items, using an order column or the SQLAlchemy OrderingList extension are better choices.
• Simple ordering: If you just need basic control over the order for a small number of related items, adding them in the desired order or custom logic on queries might suffice.