求SQLAlchemy模型字段全部可用选项及默认行为列表（原Django开发者）

Hey there! Since you’re coming from Django and diving into Flask with SQLAlchemy, I totally get wanting that clear, Django-style breakdown of field options. Let’s walk through the key ones you’ll use regularly, along with their defaults and behaviors:

Core Common Options #

These apply to nearly all field types:

• nullable: Controls whether the field can store NULL values in the database.
  • Default: True
  • Example: Your email = db.Column(db.String(128), nullable=False) makes this field required at the database level.
• unique: Ensures every value in the field is distinct across the entire table.
  • Default: False
  • Example: Pairing this with nullable=False for emails prevents duplicate user accounts with the same address.
• default: Sets a fallback value for the field when creating a new record.
  • Default: None
  • Can be a static value (e.g., default="guest"), a database function (like db.func.current_timestamp() for timestamps), or any callable that returns a valid value.
• onupdate: Defines a value to automatically set whenever the record is updated.
  • Default: None
  • Example: Your date_modified field uses this to refresh the timestamp on every edit.
• primary_key: Marks the field as the unique identifier for records in the table.
  • Default: False
  • You can set this on multiple fields to create a composite primary key.
• index: Creates a database index for the field to speed up filter/sort queries.
  • Default: False
  • Great for fields you’ll frequently use in WHERE clauses (like email for user lookups).
• comment: Adds a human-readable description to the database column for documentation purposes.
  • Default: None
  • Example: comment="User's primary contact email"

Type-Specific Options #

Some field types have extra options tailored to their data type:

• For String fields:
  • length: Mandatory for most databases, defines the maximum character count (you used this with String(128)).
• For DateTime fields:
  • timezone: If set to True, stores timezone-aware datetime values instead of naive ones.
    • Default: False
• For Integer fields:
  • autoincrement: Enables auto-incrementing numeric values. Defaults to True if the field is a primary key, otherwise False.
• For ForeignKey fields:
  • ondelete: Specifies behavior when the referenced record is deleted (e.g., ondelete="CASCADE" deletes related records, ondelete="SET NULL" sets the foreign key to NULL if nullable=True).
  • onupdate: Defines behavior when the referenced record’s primary key is updated.

Default Behaviors Recap #

A quick cheat sheet for what’s enabled by default:

• Almost all fields allow NULL values unless you set nullable=False.
• Fields are not unique by default.
• No default or auto-update values are set unless you explicitly define them.
• Primary keys auto-increment only if you use db.Integer(primary_key=True).
• No database indexes are created automatically—you have to set index=True manually.

Quick Django-to-SQLAlchemy Mapping #

Since you’re familiar with Django, here’s a quick translation to help you transition:

• Django’s blank (form validation) doesn’t have a direct SQLAlchemy equivalent, but nullable matches Django’s null (database-level nullability).
• unique works exactly the same in both frameworks.
• default supports static values and callables just like in Django.
• Django’s auto_now_add ≈ SQLAlchemy’s default=db.func.current_timestamp().
• Django’s auto_now ≈ SQLAlchemy’s onupdate=db.func.current_timestamp().

That covers the main options you’ll encounter day-to-day. If you hit edge cases or need deeper details on a specific field type, you can dive into the SQLAlchemy core docs—but this should give you that concise, Django-style reference you were looking for!

内容的提问来源于stack exchange，提问作者Kishor Pawar