Querying with filter and filter_by in Flask - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the difference between filter() and filter_by() in Flask SQLAlchemy?

filter() accepts SQL expressions and allows complex queries using operators, while filter_by() uses keyword arguments for simple equality checks.

beginner

How do you write a query to find all users with the name 'Alice' using filter_by()?

Use User.query.filter_by(name='Alice').all() to get all users named Alice.

beginner

Write a query using filter() to find users older than 30.

Use User.query.filter(User.age > 30).all() to get users older than 30.

intermediate

Can filter_by() handle complex conditions like age > 30?

No, filter_by() only supports simple equality checks. Use filter() for complex conditions.

beginner

What does .all() do at the end of a query?

.all() runs the query and returns a list of all matching records.

Which method allows you to write complex SQL expressions in Flask queries?

A filter()

B filter_by()

C all()

D get()

filter() accepts SQL expressions for complex queries, unlike filter_by() which uses simple keyword arguments.

How would you find all users with the email '[e-mail]' using filter_by()?

A User.query.filter(email='[e-mail]').all()

B User.query.get(email='[e-mail]')

C User.query.filter_by(email='[e-mail]').all()

D User.query.all(email='[e-mail]')

filter_by() uses keyword arguments for simple equality filters.

What will User.query.filter(User.age > 25).all() return?

A Users younger than 25

B Users exactly 25 years old

C All users

D Users older than 25

The query filters users where age is greater than 25.

Which method is NOT suitable for filtering by multiple conditions like name='Bob' and age > 20?

A filter_by()

B filter(User.name=='Bob', User.age>20)

C filter().filter()

D filter()

filter_by() only supports simple equality and cannot handle multiple or complex conditions.

What does the all() method do after a query?

A Deletes all matching records

B Returns all matching records as a list

C Returns the first matching record

D Counts all matching records

all() executes the query and returns a list of all results.

Explain how to use filter() and filter_by() in Flask SQLAlchemy queries. When would you choose one over the other?

Think about the complexity of the condition you want to apply.

Describe how to find all users older than 30 using Flask SQLAlchemy. Include which method you use and why.

Remember which method supports operators like >.

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