Filtered many_to_many Relationships with metadata on join Resource

Elixir Framework Forums Ash Forum
,
1

I’m building a point of sale system that supports multiple Stores. Stores share global Product, Variant and Modifier definitions.

Since prices can be different per store I have a Join Resource between Variant and Store and Modifier and Store which each have an Attribute that stores price info.

Typically when reading this data I’m only interested in seeing the set of data (in this specific instance price data) related to a specific store. So, ideally I could read all data in this hierarchy for a specific Store.

Is there a way to load these relationships so that they load the data specific to a single Store?

I know you can filter on relationships, this would likely be a custom read action? How would you make that action get used by graphql resolvers, for instance? Or for when Store is read with load that loads those?

2

Hmm…I generally suggest trying to avoid making special actions that load relationships with special filters, because that breaks a lot of things down the line. You can do it on demand pretty easily with things like: load(real: Ash.Query.filter(Related, store_id == ^store_id)), and that is something that is also supported by AshGraphql automatically, i.e you’d be able to do this:

getStore(id: "id"){
  relationship(filter: {....}) {...}
}

Often to solve for this you expose the underlying has_many relationship by defining it yourself and pointing the many_to_many to use it, i.e

has_many :foo_joins, FooJoin do
  ...
end

many_to_many :foo, Foo do
  join_relationship :foo_joins
  ...
end

Then clients can easily filter on join attributes or on destination attributes.

Ultimately, if you decide you want something special here my suggestion would be to go with a calculation that returns instances of a resource. For example:

calculate :stuff_you_want, {:array, :struct}, CalculationModule do
  constraints [items: [instance_of: SomeResource]]
end
2 Likes