Column types and constraints in Flask - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a string column named 'username' in a Flask SQLAlchemy model.

Flask

username = db.Column(db.[1](50), nullable=False)

Drag options to blanks, or click blank then click option'

AInteger

BString

CBoolean

DFloat

Attempts:

3 left

💡 Hint

Common Mistakes

Using Integer or Boolean instead of String for text data.

Forgetting to specify max length for String.

The String type defines a text column with a max length. Here, 50 means max 50 characters.

2fill in blank

medium

Complete the code to add a primary key constraint to the 'id' column.

Flask

id = db.Column(db.Integer, [1]=True)

Drag options to blanks, or click blank then click option'

Aunique

Bnullable

Cprimary_key

Dindex

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'unique=True' instead of 'primary_key=True'.

Forgetting to set primary_key on the id column.

The primary_key=True marks this column as the table's primary key.

3fill in blank

hard

Fix the error in the code to make the 'email' column unique and not nullable.

Flask

email = db.Column(db.String(120), unique=[1], nullable=[2])

Drag options to blanks, or click blank then click option'

ATrue

BFalse

CNone

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Setting nullable=True which allows empty emails.

Not setting unique=True to prevent duplicates.

Both unique and nullable should be set to True and False respectively to enforce uniqueness and non-null values. Here, unique=True and nullable=False.

4fill in blank

hard

Fill both blanks to define a column 'age' that stores integers and cannot be null.

Flask

age = db.Column(db.[1], [2]=False)

Drag options to blanks, or click blank then click option'

AInteger

BString

Cnullable

Dprimary_key

Attempts:

3 left

💡 Hint

Common Mistakes

Using String type for age.

Forgetting to set nullable=False.

The 'age' column should be an Integer type and must not allow null values, so nullable=False.

5fill in blank

hard

Fill all three blanks to define a 'created_at' column with DateTime type, defaulting to current time, and not nullable.

Flask

created_at = db.Column(db.[1], default=[2], [3]=False)

Drag options to blanks, or click blank then click option'

ADateTime

Bfunc.now()

Cnullable

DString

Attempts:

3 left

💡 Hint

Common Mistakes

Using String instead of DateTime.

Not setting default to current time.

Allowing nullable=True which can cause missing timestamps.

The 'created_at' column uses DateTime type, defaults to the current time using func.now(), and cannot be null with nullable=False.

Practice

1. Which SQLAlchemy column type should you use in Flask to store text data of variable length?

easy

A. db.String

B. db.Integer

C. db.Boolean

D. db.DateTime

Solution

1. Step 1: Understand column types for text data

   Text data of variable length is best stored using a string type that allows flexible length.

2. Step 2: Match the correct SQLAlchemy type

   db.String is designed for variable-length text, unlike db.Integer or db.Boolean.

3. Final Answer:

   db.String -> Option A

4. Quick Check:

   Variable-length text = db.String [OK]

Hint: Text data uses db.String type in Flask models [OK]

Common Mistakes:

• Choosing db.Integer for text data
• Using db.Boolean for strings
• Confusing db.DateTime with text types

2. Which of the following is the correct way to declare a non-nullable integer column named age in a Flask model?

easy

A. age = db.Column(db.Integer, nullable=True)

B. age = db.Column(db.Boolean, nullable=False)

C. age = db.Column(db.String, nullable=False)

D. age = db.Column(db.Integer, nullable=False)

Solution

1. Step 1: Identify the correct type for age

   Age should be stored as an integer, so db.Integer is correct.

2. Step 2: Set the nullable constraint

   To make the column non-nullable, use nullable=False.

3. Final Answer:

   age = db.Column(db.Integer, nullable=False) -> Option D

4. Quick Check:

   Integer + non-nullable = age = db.Column(db.Integer, nullable=False) [OK]

Hint: Use nullable=False to prevent empty values [OK]

Common Mistakes:

• Setting nullable=True when non-null is needed
• Using wrong data type like String for age
• Confusing Boolean with Integer type

3. Given this Flask model column declaration:

email = db.Column(db.String(120), unique=True, nullable=False)

What does the unique=True constraint do?

medium

A. Prevents duplicate email values in the table

B. Makes the email column optional

C. Allows multiple rows to have the same email

D. Automatically converts emails to lowercase

Solution

1. Step 1: Understand the unique constraint

   The unique=True constraint ensures no two rows have the same value in this column.

2. Step 2: Apply this to the email column

   This means duplicate emails are not allowed, keeping each email distinct.

3. Final Answer:

   Prevents duplicate email values in the table -> Option A

4. Quick Check:

   unique=True means no duplicates [OK]

Hint: unique=True means no two rows share the same value [OK]

Common Mistakes:

• Thinking unique=True allows duplicates
• Confusing unique with nullable
• Assuming unique changes data format

4. What is wrong with this Flask model column declaration?

price = db.Column(db.String, nullable=False, unique='yes')

medium

A. The nullable argument cannot be used with unique

B. The unique argument should be a boolean, not a string

C. The column type db.String cannot be used for price

D. The column name price is a reserved keyword

Solution

1. Step 1: Check the unique argument type

   The unique parameter expects a boolean value (True or False), not a string.

2. Step 2: Identify the error in the code

   Passing unique='yes' is invalid and will cause an error.

3. Final Answer:

   The unique argument should be a boolean, not a string -> Option B

4. Quick Check:

   unique must be True or False, not 'yes' [OK]

Hint: unique must be True/False, never a string [OK]

Common Mistakes:

• Using string values for boolean parameters
• Thinking nullable and unique conflict
• Assuming price is reserved keyword

5. You want to create a Flask model column for a user's phone number that must be exactly 10 digits, unique, and cannot be empty. Which declaration is best?

hard

A. phone = db.Column(db.String, unique=False, nullable=False)

B. phone = db.Column(db.Integer, unique=True, nullable=False)

C. phone = db.Column(db.String(10), unique=True, nullable=False)

D. phone = db.Column(db.String(15), nullable=True)

Solution

1. Step 1: Choose the correct type for phone numbers

   Phone numbers often contain leading zeros, so db.String(10) is better than db.Integer.

2. Step 2: Apply constraints for uniqueness and non-null

   Use unique=True to prevent duplicates and nullable=False to disallow empty values.

3. Final Answer:

   phone = db.Column(db.String(10), unique=True, nullable=False) -> Option C

4. Quick Check:

   String(10) + unique + non-null = phone = db.Column(db.String(10), unique=True, nullable=False) [OK]

Hint: Use String with length, unique=True, nullable=False for fixed phone numbers [OK]

Common Mistakes:

• Using Integer type losing leading zeros
• Allowing nullable=True when value is required
• Ignoring unique constraint for phone numbers