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'.

Practice

1. What is the main difference between filter and filter_by in Flask querying?

easy

A. filter is used only for deleting records; filter_by is for selecting.

B. filter_by allows complex expressions; filter uses simple keyword arguments.

C. filter allows complex expressions; filter_by uses simple keyword arguments.

D. filter and filter_by are exactly the same in usage.

Solution

1. Step 1: Understand filter usage

   filter accepts expressions like User.age > 20, allowing detailed conditions.

2. Step 2: Understand filter_by usage

   filter_by uses simple keyword arguments like filter_by(name='John') for equality checks.

3. Final Answer:

   filter allows complex expressions; filter_by uses simple keyword arguments. -> Option C

4. Quick Check:

   filter = complex, filter_by = simple keyword [OK]

Hint: Remember: filter = expressions, filter_by = simple equals [OK]

Common Mistakes:

• Thinking filter_by supports complex expressions
• Confusing filter_by with filter in syntax
• Assuming both methods behave identically

2. Which of the following is the correct syntax to filter users with age 30 using filter_by?

easy

A. User.query.filter_by('age == 30').all()

B. User.query.filter_by(age=30).all()

C. User.query.filter_by(age == 30).all()

D. User.query.filter_by(age > 30).all()

Solution

1. Step 1: Check filter_by syntax

   filter_by accepts keyword arguments like age=30, not expressions or strings.

2. Step 2: Identify correct usage

   Only User.query.filter_by(age=30).all() uses correct keyword argument syntax.

3. Final Answer:

   User.query.filter_by(age=30).all() -> Option B

4. Quick Check:

   filter_by uses keyword arguments [OK]

Hint: Use keyword arguments without quotes or operators in filter_by [OK]

Common Mistakes:

• Passing expressions or strings to filter_by
• Using comparison operators inside filter_by
• Confusing filter_by with filter syntax

3. Given the model User with fields name and age, what will this query return?

users = User.query.filter(User.age > 25).filter_by(name='Alice').all()

medium

A. All users named Alice older than 25

B. All users older than 25, ignoring name

C. All users named Alice, ignoring age

D. Syntax error due to mixing filter and filter_by

Solution

1. Step 1: Understand chaining filters

   Chaining filter and filter_by combines conditions with AND logic.

2. Step 2: Analyze conditions

   filter(User.age > 25) filters age > 25, filter_by(name='Alice') filters name = 'Alice'. Both apply together.

3. Final Answer:

   All users named Alice older than 25 -> Option A

4. Quick Check:

   Chained filters combine conditions [OK]

Hint: Chained filters combine all conditions with AND [OK]

Common Mistakes:

• Thinking filter_by overrides filter
• Assuming only one filter applies
• Believing mixing filter and filter_by causes error

4. What is wrong with this query?

users = User.query.filter_by(User.age > 20).all()

medium

A. filter_by requires a list, not a condition

B. User.age > 20 is a valid filter_by argument

C. The query is correct and returns users older than 20

D. filter_by does not accept expressions, only keyword arguments

Solution

1. Step 1: Check filter_by argument type

   filter_by only accepts keyword arguments like age=20, not expressions.

2. Step 2: Identify error in usage

   Passing User.age > 20 is an expression, invalid for filter_by.

3. Final Answer:

   filter_by does not accept expressions, only keyword arguments -> Option D

4. Quick Check:

   filter_by = keyword args only [OK]

Hint: filter_by needs simple key=value pairs, not expressions [OK]

Common Mistakes:

• Passing expressions to filter_by
• Confusing filter_by with filter syntax
• Expecting filter_by to parse conditions

5. You want to find all users whose name starts with 'J' and are younger than 40. Which query correctly uses filter and filter_by together?

hard

A. User.query.filter(User.name.like('J%')).filter(User.age < 40).all()

B. User.query.filter_by(name='J%').filter_by(age < 40).all()

C. User.query.filter(User.name.like('J%')).filter_by(age=40).all()

D. User.query.filter_by(name.like('J%')).filter(User.age < 40).all()

Solution

1. Step 1: Use filter for complex conditions

   Checking if name starts with 'J' requires like('J%'), which is an expression needing filter.

2. Step 2: Use filter for age less than 40

   Age < 40 is also an expression, so filter is needed again. filter_by cannot handle expressions.

3. Step 3: Verify option correctness

   User.query.filter(User.name.like('J%')).filter(User.age < 40).all() uses filter twice with correct expressions. Others misuse filter_by or syntax.

4. Final Answer:

   User.query.filter(User.name.like('J%')).filter(User.age < 40).all() -> Option A

5. Quick Check:

   Use filter for expressions, filter_by for simple equals [OK]

Hint: Use filter for expressions like like() and comparisons [OK]

Common Mistakes:

• Using filter_by with expressions like like() or <
• Mixing filter_by with invalid syntax
• Assuming filter_by can handle partial matches