Repository pattern for data access in Flask - Step-by-Step Execution

Concept Flow - Repository pattern for data access

Start Application

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Controller calls Repository

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Repository handles Data Access

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Repository queries Database

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Database returns Data

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Repository returns Data to Controller

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Controller sends Response

The controller asks the repository for data. The repository talks to the database and returns data back. The controller then sends the response.

Execution Sample

Flask

class UserRepository:
    def get_user(self, user_id):
        return User.query.filter_by(id=user_id).first()

user_repo = UserRepository()
user = user_repo.get_user(1)

This code shows a repository class fetching a user by ID from the database.

Execution Table

Step	Action	Evaluation	Result
1	Create UserRepository instance	user_repo = UserRepository()	user_repo is ready
2	Call get_user with user_id=1	user_repo.get_user(1)	Query User table for id=1
3	Database filters users where id=1	User.query.filter_by(id=1).first()	Returns user object or None
4	Return user object to caller	return user	user variable holds user data
5	Use user data in controller	user = user_repo.get_user(1)	user available for response

💡 Finished fetching user data, repository returns control to controller

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
user_repo	None	UserRepository instance	UserRepository instance	UserRepository instance
user_id	None	1	1	1
user	None	User object or None	User object or None	User object or None

Key Moments - 2 Insights

Why does the controller not access the database directly?

The controller calls the repository (see execution_table step 2) to keep data access separate and organized.

What happens if no user is found with the given ID?

The repository returns None (see execution_table step 3), so the controller can handle missing data safely.

Visual Quiz - 3 Questions

Look at the execution table, what does step 3 do?

A Creates a new user in the database

B Filters the User table for a user with id=1

C Deletes a user from the database

D Returns the controller response

💡 Hint

Check the 'Evaluation' column in step 3 for the database query action

At which step does the repository return data to the controller?

A Step 4

B Step 2

C Step 3

D Step 5

💡 Hint

Look at the 'Action' and 'Result' columns to find when data is returned

If the user ID passed was 5 instead of 1, how would the execution table change?

A Step 4 would return a different repository

B Step 3 would filter by id=1 still

C Step 2 would call get_user(5) instead of get_user(1)

D No changes in the execution table

💡 Hint

Check the 'Action' column in step 2 for the method call parameter

Concept Snapshot

Repository pattern separates data access from business logic.
Controller calls repository methods.
Repository queries database and returns data.
Keeps code organized and easier to maintain.
Example: user_repo.get_user(user_id) fetches user data.

Full Transcript

The repository pattern helps organize code by separating data access from the controller. The controller calls the repository to get data. The repository runs database queries and returns results. This keeps the controller simple and focused on handling requests and responses. In the example, the UserRepository class has a method get_user that queries the User table for a user by ID. The controller creates a UserRepository instance and calls get_user(1). The repository queries the database and returns the user object or None if not found. This pattern makes the code easier to read, test, and maintain.