Constant DEFAULT_FN 

pub const DEFAULT_FN: ColumnMarker;

Specifies a function to generate default values at runtime.

The function is called for each insert when no value is provided.

Example

#[column(default_fn = Uuid::new_v4)]
id: Uuid,

Difference from DEFAULT

• default_fn: Calls the function at runtime for each insert (e.g., UUID generation)
• default: Uses a fixed compile-time value