A mixin for the LoopBack framework that adds calculated properties to a model.

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This is a mixin for the LoopBack framework that adds calculated properties to a model.
A calculated property is a property of which the value is set once just before it is persisted to the data source.
The mixin enables you to define a callback that will be run in the before save operation hook to calculate and set the value for a given property.
npm install --save loopback-ds-calculated-mixin
Add the mixins property to your server/model-config.json:
  "_meta": {
    "sources": [
    "mixins": [
To use with your Models add the mixins attribute to the definition object of your model config.
The property you want to calculate has to be defined in the model. The callback can be a promise too.
  "name": "Item",
  "properties": {
    "name": "String",
    "description": "String",
    "status": "String",
    "readonly": "boolean"
  "mixins": {
    "Calculated": {
      "properties": {
        "readonly": "calculateReadonly",
        "isTest": {
          "recalculateOnUpdate": true,
          "callback": "calculateIsTest"  

On your model you have to define the callback methods.
// Set an item to readonly if status is 'archived'.
Item.calculateReadonly = function calculateReadonly(item) {
  return item.status === 'archived';
// Set an isTest if name is 'test'.
Item.calculateIsTest = function calculateIsTest(item) {
  return item.name === 'test'
When saving a new model instance, your callback will be called automatically. The property value will be set to the value that your callback returns.
By default, the calculated mixin is only called when saving a new instance. You can make it run on updates in addition by setting recalculateOnUpdate in the mixin config for your property.
If you set skipCalculated to true when creating or updating a model instance, the mixin will not run:
  name: 'Bilbo',
  status: 'active'
  readonly: true,
  isTest: true
}, { skipCalculated: true })
updateAll, is not currently supported as this could result in a large amount database queries and excessive memory usage. If you need to recalculate values en-mass our recommendation is to first apply your changes using updateAll, and then use the same where query to retrieve the result set in batches and apply your calculations manually.
Run the tests in test.js
npm test
Run with debugging output on:
DEBUG='loopback:mixin:calculated' npm test