Flask Database Queries

How do you perform basic queries in Flask-SQLAlchemy?

In Flask-SQLAlchemy, basic queries are performed using the query attribute on the model class. The most common methods are all() to retrieve all records and first() to retrieve the first record that matches the query.

Example of querying all records:

users = User.query.all()

Example of querying the first record that matches the filter:

user = User.query.filter_by(username='john').first()

In this example, all() retrieves all the users from the User table, and filter_by() with first() retrieves the first user with the username "john".

How do you filter data using filter() and filter_by() in Flask-SQLAlchemy?

The filter() method allows you to write more complex SQL-like queries using comparison operators, while filter_by() allows you to filter based on keyword arguments.

Example of using filter():

users = User.query.filter(User.age > 18).all()

Example of using filter_by():

user = User.query.filter_by(username='john').first()

In these examples, filter() is used to retrieve users older than 18, and filter_by() is used to find the user with the username "john".

How do you order query results in Flask-SQLAlchemy?

To order query results, you can use the order_by() method in Flask-SQLAlchemy. By default, order_by() sorts the results in ascending order. You can use desc() for descending order.

Example of ordering results in ascending order:

users = User.query.order_by(User.username).all()

Example of ordering results in descending order:

users = User.query.order_by(User.username.desc()).all()

In these examples, users are ordered by their usernames in ascending and descending order, respectively.

How do you limit the number of query results in Flask-SQLAlchemy?

You can limit the number of query results using the limit() method. This method restricts the number of rows returned by the query.

Example of limiting query results:

users = User.query.limit(5).all()

In this example, only the first five users are retrieved from the database.

How do you count the number of query results in Flask-SQLAlchemy?

You can count the number of records returned by a query using the count() method. This method returns the total number of rows that match the query.

Example of counting the number of users:

user_count = User.query.count()

In this example, the total number of users in the User table is counted and stored in the user_count variable.

How do you paginate query results in Flask-SQLAlchemy?

To paginate query results, you can use the paginate() method. This method splits the results into pages, allowing you to fetch a specific page of data.

Example of paginating query results:

page = 1
users = User.query.paginate(page=page, per_page=10).items

In this example, the first page of users is retrieved, with 10 users per page. The items attribute contains the users on the current page.

How do you perform joins in Flask-SQLAlchemy?

You can perform SQL joins in Flask-SQLAlchemy using the join() method. This method allows you to combine rows from two or more tables based on a related column.

Example of performing a join between User and Post models:

posts = db.session.query(Post).join(User).filter(User.username == 'john').all()

In this example, the Post model is joined with the User model, and the posts by the user "john" are retrieved.

How do you perform group by queries in Flask-SQLAlchemy?

The group_by() method in Flask-SQLAlchemy allows you to group query results by one or more columns, similar to the SQL GROUP BY clause.

Example of grouping posts by user:

from sqlalchemy import func

post_counts = db.session.query(User.username, func.count(Post.id)).join(Post).group_by(User.username).all()

In this example, posts are grouped by the username, and the number of posts per user is counted using func.count().

How do you update records using queries in Flask-SQLAlchemy?

To update records, you first query the records you want to update, modify their attributes, and then commit the changes using db.session.commit().

Example of updating user information:

user = User.query.filter_by(username='john').first()
user.email = '[email protected]'
db.session.commit()

In this example, the email of the user with the username "john" is updated, and the changes are committed to the database.

How do you delete records using queries in Flask-SQLAlchemy?

To delete records, you query the record, delete it using db.session.delete(), and commit the changes using db.session.commit().

Example of deleting a user:

user = User.query.filter_by(username='john').first()
db.session.delete(user)
db.session.commit()

In this example, the user with the username "john" is deleted from the database.

How do you execute raw SQL queries in Flask-SQLAlchemy?

If needed, you can execute raw SQL queries in Flask-SQLAlchemy using the db.engine.execute() method. This is useful when you need to perform complex queries that are not easily handled by SQLAlchemy's ORM methods.

Example of executing a raw SQL query:

result = db.engine.execute("SELECT * FROM user WHERE username='john'")
for row in result:
    print(row)

In this example, a raw SQL query is executed to select all data for the user with the username "john". The results are then iterated over to print each row.

What is eager loading, and how do you use it in Flask-SQLAlchemy?

Eager loading is a technique used to load related objects along with the main object in a single query. In Flask-SQLAlchemy, you can use joinedload() or subqueryload() to eagerly load relationships and avoid multiple database queries.

Example of eager loading using joinedload():

from sqlalchemy.orm import joinedload

users = User.query.options(joinedload(User.posts)).all()

In this example, the user's posts are eagerly loaded along with the user records, reducing the number of queries needed to fetch related posts.

What is lazy loading in Flask-SQLAlchemy?

Lazy loading is a technique where related objects are loaded only when they are accessed. This can result in multiple queries being sent to the database as each related object is accessed, but it reduces the initial query time.

Example of lazy loading:

user = User.query.get(1)
print(user.posts)  # Posts are loaded only when accessed

In this example, the user's posts are not loaded until they are explicitly accessed, resulting in a separate query to fetch the posts.