Reads a field directly from an AssignmentRelationship record.

Filters AssignmentRelationship by target_id = current.id and returns the named field from each matching record — no second hop to the source instance. This is the right choice when you want a value that lives on the relationship itself (:value, :thing, :pool, :alias) rather than on the assigning instance.

Use FieldViaAssignedRelationship instead when you need a field from the source instance (e.g. :name).

Options

• field: (required) — atom naming the field to read from the relationship record (e.g. :value, :thing, :pool, :alias).
• alias: (optional) — atom matching the alias attribute on the relationship. When omitted, all assignments where target_id = current.id are included.

Examples

# Port number assigned to this service under the :primary slot
calculate :assigned_port, {:array, :integer},
  {Diffo.Provider.Calculations.FieldFromAssignment, [alias: :primary, field: :value]}

# Pool name for every assignment on this instance
calculate :assignment_pools, {:array, :atom},
  {Diffo.Provider.Calculations.FieldFromAssignment, [field: :pool]}