Querying with filter and filter_by in Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this Flask-SQLAlchemy query return?

Consider a model User with fields id and name. What will this query return?

users = User.query.filter(User.name == 'Alice').all()

A A single User object with name 'Alice', or None if not found.

B A list of all User objects whose name is exactly 'Alice'.

C A list of all User objects regardless of name.

D Raises a syntax error because filter needs a string argument.

💡 Hint

Remember that filter returns a query object that can return multiple results with .all().

The filter method takes a condition and returns a query for all matching rows. Using .all() returns a list of all User objects with name 'Alice'.

📝 Syntax

intermediate

Which option correctly uses filter_by to find users named 'Bob'?

Given the User model with a name field, which query correctly finds all users named 'Bob'?

A User.query.filter_by(name='Bob').all()

B User.query.filter_by('name' == 'Bob').all()

C User.query.filter_by(name == 'Bob').all()

D User.query.filter_by(name='Bob')()

💡 Hint

filter_by uses keyword arguments, not expressions or strings.

filter_by accepts keyword arguments like name='Bob'. It does not accept expressions or strings as conditions.

🔧 Debug

advanced

Why does this filter query raise an error?

What error does this code raise and why?

users = User.query.filter(User.age > 20).filter_by(name='Charlie').all()

A No error; returns users older than 20 named 'Charlie'.

B AttributeError because 'User' has no attribute 'age'.

C InvalidRequestError because mixing filter and filter_by with conflicting conditions.

D TypeError because filter_by cannot be chained after filter.

💡 Hint

filter and filter_by can be chained; filter_by adds keyword filters.

Chaining filter and filter_by is valid. The query returns users with age > 20 and name 'Charlie'. No error occurs if fields exist.

❓ state_output

advanced

What is the length of the result list?

Assuming the database has users with names: ['Anna', 'Bob', 'Anna', 'Charlie'], what is the length of the list returned by:

result = User.query.filter_by(name='Anna').all()

A 1

B 0

C 3

D 2

💡 Hint

filter_by returns all matching rows, not just one.

There are two users named 'Anna', so the list length is 2.

🧠 Conceptual

expert

Which statement about filter and filter_by is true?

Choose the correct statement about the difference between filter and filter_by in Flask-SQLAlchemy.

A filter_by accepts SQL expressions, filter accepts only keyword arguments.

B Both filter and filter_by accept only strings as conditions.

C filter_by accepts keyword arguments, filter accepts SQL expressions.

D filter_by can be used only once per query, filter can be chained multiple times.

💡 Hint

Think about how you write conditions in each method.

filter_by uses keyword arguments like name='Alice'. filter uses SQL expressions like User.name == 'Alice'.