Feature Request: Add support for eqFn in relation definitions for dynamic matching

[damienroche]

Currently, relations between entities are defined using a declarative structure like the following:

defineModel<>({
   name: 'itinerary',
   relations: {
     points: {
       to: {
         itineraryPoints: {
           on: 'itinerary',
           eq: 'id'
         }
       },
       many: true
     }
   }
})

This works well for simple key-to-key equality (e.g., item.itinerary === target.id).

However, it becomes limiting when more complex logic is required — for example, when the source entity contains an array of IDs, or when the relation involves custom business logic.

Proposed Enhancement:

Introduce a new field, eqFn, as an alternative to the on + eq pair.
eqFn would allow defining a custom function to determine whether a given item matches a target:

relations: {
  points: {
    to: {
      itineraryPoints: {
        eqFn: (item, target) => item.itineraryPointIds.includes(target.id)
      }
    },
    many: true
  }
}

Details

• eqFn is exclusive with on and eq — when eqFn is present, the others are ignored.
• This provides greater flexibility while keeping the existing declarative approach for simpler cases.
• Fully backward compatible: current usage of on + eq remains unchanged.

What do you think of this feature?

[Akryum]

This looks good, but I'm not sure about the name of the function. Maybe find or filter?

[damienroche]

@Akryum for sure. I think find is the best one.