Loopback ctx.instance vs ctx.data

Question

I am not able to understand the difference between Ctx.instance and Ctx.data in loopback 'before save ' callback.

Answer 1

The difference is described in our documentation, see Operation hooks >> before save. Cross-posting the relevant content here:

Depending on which method triggered this hook, the context will have one of the following sets of properties:

• Full save of a single model

  • (...)
  • instance - the model instance to be saved. The value is an instance of Model class.

• Partial update of possibly multiple models

  • (...)
  • data - the (partial) data to apply during the update
  • currentInstance - the affected instance.

When you call Customer.create({/*...*/}), the hook is invoked with ctx.instance set to the instance of your model with all properties populated from the object passed to create():

{
  name: 'Miroslav',
  isPreferred: false,
  // etc.
}       

When you call customer.patchAttributes({isPreferred: true}), the hook is invoked with ctx.data containing only the properties provided to patchAttributes().

{
  isPreferred: true
}

You can learn more in the source code, see loopback-datasource-juggler/lib/dao.js.