elixir - Ecto table join -

the ecto documentation shows how join 2 tables (a<-b) based on association defined in schema. i'd expand adding table (b <- c)

https://hexdocs.pm/ecto/ecto.html#assoc/2

but error. how reflect fact :comments belong :posts in query?

repo.all u in app.user,                                                                                                                                                                  join: p in assoc(u, :posts),                                                                                                                                                                    join: c in assoc(p, :comments),                                                                                                                                                                 preload: [posts: p],                                                                                                                                                                            preload: [comments: c]                                                                                                                                                                          ** (ecto.queryerror) field `app.user.comments` in preload not association in query:                                                                                                           u in app.user,                                                                                                                                                                                    join: p in app.post,                                                                                                                                                                                 on: p.user_id == u.id,                                                                                                                                                                                   join: c in app.comment,                                                                                                                                                                             on: c.post_id == p.id,                                                                                                                                                                                   select: u,                                                                                                                                                                                               preload: [posts: p, comments: c]                                                                                                                                                                        (elixir) lib/enum.ex:651: enum."-each/2-lists^foreach/1-0-"/2                                                                                                                                            (elixir) lib/enum.ex:651: enum.each/2                                                                                                                                                                      (ecto) lib/ecto/repo/queryable.ex:119: ecto.repo.queryable.execute/5                                                                                                                                     (ecto) lib/ecto/repo/queryable.ex:40: ecto.repo.queryable.all/4  

you can preload associations in tree-like structure , map results needed.

> subquery = app.post |> preload(:comments) #ecto.query<from p in app.post, preload: [:comments]>  > query = app.user |> preload(posts: ^subquery) #ecto.query<from u in app.user,  preload: [posts: #ecto.query<from p in app.post, preload: [:comments]>]>  > repo.all(query) [   %app.user{...},   %app.user{...},   %app.user{     ...,     posts: [       %app.post{...},       %app.post{...},       %app.post{         ...,         comments: [           %app.comment{...},           %app.comment{...},           %app.comment{...}         ]       }     ]   } ]