Migrating Motoko

Syntax

Parenthetical expression immediately before the actor:

import Migration "migration";

(with migration = Migration.run)
actor {
  var newState : Float = 0.0;
};

Or inline:

import Int "mo:core/Int";

(with migration = func(old : { var state : Int }) : { var newState : Float } {
  { var newState = old.state.toFloat() }
})
actor {
  var newState : Float = 0.0;
};

Or using the shorthand when the imported module exports a migration field:

import { migration } "migration";

(with migration)
actor { ... };

Migration Function Rules

• Type: func (old : { ... }) : { ... } — local, non-generic, both records must use persistable types (no functions or mutable arrays)
• Domain: old actor fields (names and types from the previous version)
• Codomain: new actor fields (must exist in the new actor with compatible types)
• Runs only on upgrade — on fresh install, initializers run normally
• If the migration traps, the upgrade is aborted and the canister stays on the old version

Field semantics

Migration Module Pattern

Keep migrations in a separate module. Define old types inline — do not import them from old code paths:

// migration.mo
import Types "types";
import Map "mo:core/Map";

module {
  type OldTask = { id : Nat; title : Text; completed : Bool };

  type OldActor = {
    var tasks : Map.Map<Nat, OldTask>;
    var nextId : Nat;
  };

  type NewActor = {
    var tasks : Map.Map<Nat, Types.Task>;
    var nextId : Nat;
  };

  public func run(old : OldActor) : NewActor {
    let tasks = old.tasks.map<Nat, OldTask, Types.Task>(
      func(_, task) {
        {
          id = task.id;
          title = task.title;
          due = 0;
          var status = if (task.completed) #completed else #pending;
        }
      }
    );
    { var tasks; var nextId = old.nextId };
  };
};

// main.mo
import Migration "migration";

(with migration = Migration.run)
actor {
  var tasks : Map.Map<Nat, Types.Task>;
  var nextId : Nat = 0;
};

Common Patterns

Add field with default

old.users.map<Nat, OldUser, NewUser>(
  func(_, u) { { u with zipCode = "" } }
)

Add optional field

{ task with var assignee = null : ?Principal }

Bool to variant

var status = if (task.completed) #completed else #pending;

Rename a field

Consume old name, produce new name:

func(old : { var state : Int }) : { var value : Int } {
  { var value = old.state }
}

Drop a field

Consume it in the input, omit from output. Compiler warns — ensure the loss is intentional.

Checklist

• Decide: implicit (compatible change) or explicit (migration function)
• If explicit: define old types inline in migration.mo
• Migration type: func (old : RecordIn) : RecordOut with persistable types
• Attach with (with migration = Migration.run) before the actor
• Do not use preupgrade/postupgrade for data migration
• Verify with mops check --fix and mops build

Additional Resources

• Load writing-motoko for general Motoko language reference
• Load migrating-motoko-enhanced for multi-migration with --enhanced-migration