Relationships (one-to-many) in Flask - Step-by-Step Execution

Concept Flow - Relationships (one-to-many)

Define Parent Model

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Define Child Model with ForeignKey

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Create Parent Instance

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Add Child Instances linked to Parent

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Query Parent

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Access Child Collection via Relationship

This flow shows how to set up a one-to-many relationship in Flask by defining models, linking children to a parent, and accessing related data.

Execution Sample

Flask

class Author(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    books = db.relationship('Book', backref='author')

class Book(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    author_id = db.Column(db.Integer, db.ForeignKey('author.id'))

Defines Author and Book models where one Author can have many Books linked by author_id foreign key.

Execution Table

Step	Action	State Change	Result
1	Define Author model with books relationship	Author.books set as collection of Book objects	Ready to link books to authors
2	Define Book model with author_id foreign key	Book.author_id links to Author.id	Books can reference their author
3	Create Author instance (author1)	author1.id assigned after commit	Author record in DB
4	Create Book instances (book1, book2) with author_id=author1.id	Books linked to author1	Book records in DB linked to author1
5	Query author1.books	Fetches list of books linked to author1	Returns [book1, book2]
6	Access book1.author	Fetches author linked to book1	Returns author1
7	Exit	All relationships established and accessible	End of flow

💡 All models defined, instances created, and relationships accessed successfully

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
author1	None	Author instance with id assigned	Same instance	Same instance
book1	None	None	Book instance with author_id=author1.id	Same instance
book2	None	None	Book instance with author_id=author1.id	Same instance
author1.books	Empty	Empty	[book1, book2]	[book1, book2]
book1.author	None	None	author1	author1

Key Moments - 3 Insights

Why do we use db.relationship in the parent model instead of just foreign keys?

db.relationship creates a convenient way to access all related child objects as a list, as shown in step 1 and step 5 of the execution_table, while foreign keys only store the link in the child.

How does Flask know which foreign key links the child to the parent?

The foreign key in the child model (author_id) explicitly references the parent's primary key (author.id), as shown in step 2, so Flask uses this to connect the models.

What happens if you access author1.books before adding any books?

The books collection will be empty, as shown in variable_tracker after step 3, because no Book instances are linked yet.

Visual Quiz - 3 Questions

Look at the execution_table at step 5, what does author1.books return?

A An empty list

B The author1 instance

C A list containing book1 and book2

D None

💡 Hint

Check the 'Result' column at step 5 in the execution_table

At which step is the foreign key author_id assigned to book1 and book2?

A Step 4

B Step 3

C Step 5

D Step 2

💡 Hint

Look at the 'Action' and 'State Change' columns in the execution_table

If you remove db.relationship from Author, what will happen when accessing author1.books?

A It will return all books linked to author1

B It will raise an AttributeError

C It will return None

D It will return author1 instance

💡 Hint

Refer to the importance of db.relationship in step 1 and step 5 of the execution_table

Concept Snapshot

One-to-many relationships in Flask use a foreign key in the child model to link to the parent.
The parent model uses db.relationship to access all related children as a list.
Create parent and child instances, assign foreign keys, then query via relationship.
This setup allows easy navigation from parent to children and vice versa.
Always define foreign keys in child and relationships in parent for clarity.

Full Transcript

In Flask, to create a one-to-many relationship, you define two models: a parent and a child. The child model has a foreign key column that points to the parent's primary key. The parent model uses db.relationship to create a collection of child objects. When you create instances, you assign the parent's id to the child's foreign key. You can then access all children from the parent using the relationship attribute. This setup helps organize related data clearly and access it easily in your application.