This is a mixin for the LoopBack framework that adds computed properties to a model.

A computed property is a property of which the value is set dynamically after reading model data from the data source.

• The mixin uses the loaded observer.
• It only runs when a single instance gets loaded, e.g. it checks ctx.instance.
• It only runs when it is a new instance, e.g. it checks ctx.isNewInstance.
• It overrides the configured property if one exists in the data source.

npm install --save loopback-ds-computed-mixin

Add the mixins property to your server/model-config.json:

{ "_meta": { "sources": [ "loopback/common/models", "loopback/server/models", "../common/models", "./models" ], "mixins": [ "loopback/common/mixins", "../node_modules/loopback-ds-computed-mixin/lib", "../common/mixins" ] } } 

To use with your Models add the mixins attribute to the definition object of your model config.

The property you want to compute 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": { "Computed": { "properties": { "readonly": "computeReadonly" } } } }

On your model you have to define the callback method.

// Set an item to readonly if status is archived Item.computeReadonly = function computeReadonly(item) { return item.status === 'archived'; };

Run the tests in test.js

 npm test

Run with debugging output on:

 DEBUG='loopback:mixin:computed' npm test