[Proposal] Support `through` via `belongs_to` association

[tom.cr...@gmail.com]

Currently the through option is only supported via has_one and has_many associations, but not via belongs_to associations. Would make sense to support this too? For example:

schema "chapter" do

  # ...

  belongs_to(:book, Book)

end

schema "page" do

  # ...

  belongs_to(:chapter, Chapter)

  belongs_to(:book, through: [:chapter, :book])

end

What do you think?

[José Valim]

You can use has_one for the exact same behaviour. belongs_to assumes that the current schema will have the foreign key for the association, which is not the case here. :)

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[tom.cr...@gmail.com]

That's what I tried initially, but if you use has_one via a belongs_to, we get the following:

(ArgumentError) schema does not have the association :chapter used by association :book, please ensure the association exists and is defined before the :through one

Stacktrace:

  │ lib/ecto/association.ex:796: Ecto.Association.HasThrough.struct/3

  │ lib/ecto/schema.ex:1707: Ecto.Schema.association/5

  │ ...

  │ (stdlib 3.13) erl_eval.erl:680: :erl_eval.do_apply/6

[tom.cr...@gmail.com]

For example:

schema "chapter" do

  # ...

  belongs_to(:book, Book)

end

schema "page" do

  # ...

  belongs_to(:chapter, Chapter)

  has_one(:book, through: [:chapter, :book]) # this gives the ArgumentError

end

[José Valim]

Perhaps the best option from now on is to make the above work. :)

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[tom.cr...@gmail.com]

Sounds good to me... should I create an issue in github or does the proposal need to be "fleshed out" a bit?

[José Valim]

Creating an issue is fine but someone would have to work on it at some point. :)

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[tom.cr...@gmail.com]

Understood. I'll take a look at it and see if it's within my meagre capabilities :)

[tom.cr...@gmail.com]

Good news, it turns out has_one via a belongs_to actually works already, but it must be defined after the belongs_to in the schema. Obvious when you think about it.

Sorry for wasting your time Jose!