Create index in Mongoose for array of objects

I have two models setup:

ShopsModel

var ShopsSchema = new mongoose.Schema({
  name: {
    type: String,
    required: true
  }
});

var ShopsModel = mongoose.model("Shops", ShopsSchema);

FieldGroupsModel

var FieldGroupsSchema = new mongoose.Schema({
  title: {
    type: String,
    required: true
  },
  fields: [{
    label: {
      type: String,
      required: true
    },
    handle: {
      type: String,
      required: true
    }
  }],
  shop: {
    type: mongoose.Schema.Types.ObjectId,
    ref: "Shops"
  }
});

var FieldGroupsModel = mongoose.model("FieldGroups", FieldGroupsSchema)

Each FieldGroups instance has a ShopsModel associated with it.

I need to create an index for the FieldGroupsModel fields[i].handle value, this index needs two rules; it needs to be unique to each FieldGroupsModel instance, so this data would be invalid:

{
  title: "Some field group title here",
  fields: [
    {
      label: "Some label 1"
      handle: "some-label-1"
    },
    {
      label: "Some label 1"
      handle: "some-label-1" // Error: `some-label-1` already exists as a value of `fields[i].handle`.
    }
  ],
  shop: {
    "$oid": "1"
  }
}

The second rule is that the first rule should only be in place for FieldGroupsModel instances which share the same shop value. So this data would be invalid:

// First bit of data
{
  title: "Some field group title here",
  fields: [
    {
      label: "Some label 1"
      handle: "some-label-1"
    }
  ],
  shop: {
    "$oid": "1"
  }
}

// Second bit of data
{
  title: "Another field group title here",
  fields: [
    {
      label: "Some label 1"
      handle: "some-label-1" // Error: `some-label-1` already exists as a value of `fields[i].handle` of a document which shares the same `shop` value.
    }
  ],
  shop: {
    "$oid": "1"
  }
}

However, this would be valid:

// First bit of data
{
  title: "Some field group title here",
  fields: [
    {
      label: "Some label 1"
      handle: "some-label-1"
    }
  ],
  shop: {
    "$oid": "1"
  }
}

// Second bit of data
{
  title: "Another field group title here",
  fields: [
    {
      label: "Some label 1"
      handle: "some-label-1" // This is valid because there's no other documents with the same `shop` value with the same `fields[i].handle` value.
    }
  ],
  shop: {
    "$oid": "2"
  }
}

I'm quite new to Mongo and Mongoose so any help here would be greatly appreciated! :)

via Tom