models: move Change LDL def into a json file

common/models/change.json

@@ -0,0 +1,25 @@
+{
+  "name": "Change",
+  "trackChanges": false,
+  "properties": {
+    "id": {
+      "type": "string",
+      "id": true
+    },
+    "rev": {
+      "type": "string"
+    },
+    "prev": {
+      "type": "string"
+    },
+    "checkpoint": {
+      "type": "number"
+    },
+    "modelName": {
+      "type": "string"
+    },
+    "modelId": {
+      "type": "string"
+    }
+  }
+}

lib/builtin-models.js

@@ -33,7 +33,9 @@ module.exports = function(loopback) {
     require('../common/models/user.json'),
     require('../common/models/user.js'));
 
-  loopback.Change = require('../common/models/change');
+  loopback.Change = createModel(
+    require('../common/models/change.json'),
+    require('../common/models/change.js'));
 
   loopback.Checkpoint = createModel(
     require('../common/models/checkpoint.json'),

lib/persisted-model.js

@@ -960,7 +960,10 @@ PersistedModel.enableChangeTracking = function() {
 }
 
 PersistedModel._defineChangeModel = function() {
-  var BaseChangeModel = require('./../common/models/change');
+  var BaseChangeModel = loopback.Change;
+  assert(BaseChangeModel,
+    'Change model must be defined before enabling change replication');
+
   return this.Change = BaseChangeModel.extend(this.modelName + '-change',
     {},
     {