How do relationships with composite keys work?

I found this mention of composite keys in the docs:

  has_many :composite_key_posts, MyApp.CompositeKeyPost do
    destination_attribute :author_id
  end

  has_many :composite_key_posts, MyApp.CompositeKeyPost do
    destination_attribute :author_id
  end

Ideally I need something like:

belongs_to :widget, MyApp.Widget do 
   source_attribute: [:scope_key, :source_key]
   destination_attribute: [:scope_key, :destination_key]
end

belongs_to :widget, MyApp.Widget do 
   source_attribute: [:scope_key, :source_key]
   destination_attribute: [:scope_key, :destination_key]
end

A similar effect to have scope_key being a tenant.

7 Replies

ZachDaniel•2y ago

There isn’t a way to do that currently, without using manual relationships

Robert GraffOP•2y ago

Very edge use case. I have some easy alternative options that will work.

Dimitri•15mo ago

While looking for an answer to this same question I ended up here. Hi btw! First message here 🙂 I found out the destination_attribute still expects a single atom. Am i correct in assuming there hasn't been any change arround this subject and a manual relationship is still the way to go when one would be trying to write a belongs_to pointing to a table with a composit primary key?

ZachDaniel•15mo ago

👋 This forum area is archived, but happy to answer here 😄 You can achieve this kind of thing with the filter option

belongs_to :widget, MyApp.Widget do
  filter expr(other_destination_field == parent(other_source_field))
end

belongs_to :widget, MyApp.Widget do
  filter expr(other_destination_field == parent(other_source_field))
end

Robert GraffOP•15mo ago

That's correct

Dimitri•15mo ago

Heya, thanks for respondin guys! I thought I'd keep it in one place and kind of missed its an actual archive 😅 Im not quite sure tho how this would work in my case. As Im trying to set an actual foreign key based of of a primary key thats a composite of 2 fields. Your example just looks like a runtime query instead of being able to save a foreignkey (in postgres) 🧐 ?

ZachDaniel•15mo ago

Ah, okay so to have the migration generator do that you can configure the references block in the postgres DSL https://hexdocs.pm/ash_postgres/1.5.15/dsl-ashpostgres-datalayer.html#postgres-references-reference-match_with

reference :widget, match_with: [other_source_field: :other_destination_field]

reference :widget, match_with: [other_source_field: :other_destination_field]

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How do relationships with composite keys work?

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