Column types and constraints in Flask

Introduction

Column types and constraints define what kind of data a database column can hold and rules it must follow. This helps keep data correct and organized.

When creating a new database table to store user information like names and emails.

When you want to make sure a column only accepts numbers or dates.

When you want to prevent duplicate entries in a column, like unique usernames.

When you want to make sure some columns always have a value (not empty).

When you want to link data between tables using keys.

Syntax

Flask

Column(column_type, constraint1, constraint2, ...)

Example:
name = Column(String(50), nullable=False, unique=True)

Column type defines the kind of data (e.g., String, Integer, Date).

Constraints are rules like nullable=False (must have value) or unique=True (no duplicates).

Examples

This creates an integer column named id that is the primary key (unique identifier).

Flask

id = Column(Integer, primary_key=True)

This creates a string column for emails that must be unique and cannot be empty.

Flask

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

This column stores date and time and must always have a value.

Flask

created_at = Column(DateTime, nullable=False)

Sample Program

This Flask app defines a User table with columns using types and constraints. It creates the table, adds one user, and prints the user info.

Flask

from flask_sqlalchemy import SQLAlchemy
from flask import Flask
from datetime import datetime

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///:memory:'
db = SQLAlchemy(app)

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(50), unique=True, nullable=False)
    email = db.Column(db.String(120), unique=True, nullable=False)
    joined_on = db.Column(db.DateTime, nullable=False, default=datetime.utcnow)

with app.app_context():
    db.create_all()
    user1 = User(username='alice', email='[e-mail]')
    db.session.add(user1)
    db.session.commit()
    print(f"User added: {user1.username}, Email: {user1.email}, Joined: {user1.joined_on}")

Important Notes

Use nullable=False to make sure a column always has data.

unique=True prevents duplicate values in a column.

Primary keys uniquely identify each row and are required for most tables.

Summary

Column types tell the database what kind of data to expect.

Constraints set rules to keep data valid and organized.

Using types and constraints helps avoid errors and keeps your data clean.

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