warn when migration did not take a callback and did not return a promise

lib/migrate.js

@@ -30,6 +30,7 @@ function migrate (set, direction, migrationName, fn) {
 
     // Run the migration function
     set.emit('migration', migration, direction)
+    var arity = migration[direction].length
     var returnValue = migration[direction](function (err) {
       if (isPromise) return set.emit('warning', 'if your migration returns a promise, do not call the done callback')
       completeMigration(err)
@@ -38,6 +39,9 @@ function migrate (set, direction, migrationName, fn) {
     // Is it a promise?
     isPromise = typeof Promise !== 'undefined' && returnValue instanceof Promise
 
+    // If not a promise and arity is not 1, warn
+    if (!isPromise && arity < 1) set.emit('warning', 'it looks like your migration did not take or callback or return a Promise, this might be an error')
+
     // Handle the promises
     if (isPromise) {
       returnValue

test/fixtures/promises/4-neither-test.js

@@ -0,0 +1,9 @@
+'use strict'
+
+module.exports.up = function () {
+  arguments[0]()
+}
+
+module.exports.down = function () {
+  arguments[0]()
+}

test/promises.js

@@ -52,8 +52,22 @@ describe('Promise migrations', function () {
     })
   })
 
+  it('should warn with no promise or callback', function (done) {
+    set.up('3-callback-promise-test.js', function () {
+      var warned = false
+      set.on('warning', function (msg) {
+        assert(msg)
+        warned = true
+      })
+      set.up('4-neither-test.js', function () {
+        assert(warned)
+        done()
+      })
+    })
+  })
+
   it('should error with rejected promises', function (done) {
-    set.up('4-failure-test.js', function (err) {
+    set.up('99-failure-test.js', function (err) {
       assert(err)
       assert.equal(err.message, 'foo')
       done()