Trait Migrate 

pub trait Migrate: TableSchema {
    // Provided methods
    fn default_value(_column: &str) -> Option<Value> { ... }
    fn transform_column(
        _column: &str,
        _old: Value,
    ) -> Result<Option<Value>, DbmsError> { ... }
}

Per-table extension hook for schema migrations.

The derive macro #[derive(Table)] emits an empty impl Migrate for T {} for every table by default, so callers only need to provide a manual impl when they tag the struct with #[migrate]. The trait extends TableSchema so implementors automatically have access to the table’s column definitions and snapshot.

Provided Methods

fn default_value(_column: &str) -> Option<Value>

Dynamic default for an MigrationOp::AddColumn operation on a non-nullable column.

Returning None falls back to the static #[default = ...] attribute declared on the column. If neither produces a value, migration aborts with MigrationError::DefaultMissing.

fn transform_column( _column: &str, _old: Value, ) -> Result<Option<Value>, DbmsError>

Transform a stored value when its column changes to an incompatible type that does not fit the framework’s widening whitelist.

• Ok(None) — no transform; the framework errors with MigrationError::IncompatibleType unless widening already applies.
• Ok(Some(v)) — use v as the new value.
• Err(_) — abort the migration; the journaled session rolls back.

Dyn Compatibility

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.

Implementors