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column-attributes.md

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Column Attributes in Flask-SQLAlchemy

When defining models in Flask-SQLAlchemy, each field is defined using db.Column with specific attributes. These attributes control the behavior and constraints of the database columns.

Common Column Attributes

1. Primary Key (``)

Defines the primary key of the table.

id = db.Column(db.Integer, primary_key=True)

2. Unique (``)

Ensures that all values in this column are unique.

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

3. Nullable (``)

Specifies whether the column can have NULL values. By default, columns are nullable.

username = db.Column(db.String(80), nullable=False)

4. Default (``)

Sets a default value for the column.

is_active = db.Column(db.Boolean, default=True)

5. Index (``)

Creates an index on the column to speed up queries.

created_at = db.Column(db.DateTime, index=True)

6. Data Types (**, **, etc.)

Defines the type of data that the column will store.

  • db.String: Variable-length string.
  • db.Integer: Integer values.
  • db.Float: Floating-point numbers.
  • db.Boolean: Boolean values.
  • db.DateTime: Date and time values.
  • db.Text: Large text fields.

7. Foreign Key (``)

Establishes relationships between tables.

profile_id = db.Column(db.Integer, db.ForeignKey('profile.id'))

8. Server Default (``)

Specifies a default value at the database level rather than application level.

status = db.Column(db.String(50), server_default='active')

9. Check Constraint (``)

Adds constraints to validate data.

__table_args__ = (
    db.CheckConstraint('age >= 18', name='check_age_positive'),
)

10. Relationships (``)

Defines relationships between tables.

profile = db.relationship('Profile', backref='user', lazy=True)

Example Model with Attributes

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(80), unique=True, nullable=False)
    email = db.Column(db.String(120), unique=True, nullable=False)
    created_at = db.Column(db.DateTime, index=True, default=datetime.utcnow)
    is_active = db.Column(db.Boolean, default=True)

    def __repr__(self):
        return f'<User {self.username}>'

Conclusion

Understanding and using column attributes properly allows you to build robust and efficient databases. In the next section, we will cover relationships and more advanced features in Flask-SQLAlchemy.

More Tips

If you don't want to limit the string length in SQLAlchemy, you have two options:

✅ Option 1: Use db.Text Instead of db.String()

db.Text stores long, unrestricted text (like passwords, descriptions, or large text data).

f_name = db.Column(db.String(30), nullable=False)  # Limited to 30 chars
l_name = db.Column(db.String(30), nullable=False)  # Limited to 30 chars
password = db.Column(db.Text, nullable=False)  # No character limit

✅ db.Text is best for long, unknown-length strings.



✅ Option 2: Use db.String() Without a Limit

If you don't provide a limit, SQLAlchemy doesn't enforce one, but the database might have a default limit.

password = db.Column(db.String(), nullable=False)  # No explicit limit

⚠️ Warning:

  • Some databases (like SQLite) might still impose an internal limit.
  • Using db.Text is safer for long values.




🔥 Which One Should You Use?

  • Use db.Text if you truly need unlimited text (like large descriptions, comments, or passwords).
  • Use db.String() without a limit if you trust the database's default behavior.