Fix through association nested preloads

integration_test/cases/joins.exs

@@ -589,4 +589,63 @@ defmodule Ecto.Integration.JoinsTest do
     assert [post] = TestRepo.all(query)
     assert post.post_user_composite_pk
   end
+
+  test "joining a through association with a nested preloads" do
+    post = TestRepo.insert!(%Post{title: "1"})
+    user = TestRepo.insert!(%User{name: "1"})
+    TestRepo.insert!(%Comment{text: "1", post_id: post.id})
+    TestRepo.insert!(%Permalink{post_id: post.id, user_id: user.id})
+
+    query =
+      from c in Comment,
+        join: pp in assoc(c, :post_permalink),
+        join: u in assoc(pp, :user),
+        preload: [post_permalink: {pp, [:post, user: u]}]
+
+    [comment] = TestRepo.all(query)
+
+    assert not Ecto.assoc_loaded?(comment.post)
+    assert %Permalink{user: %User{}, post: %Post{}} = comment.post_permalink
+  end
+
+  test "joining multiple through associations with a nested preloads" do
+    post = TestRepo.insert!(%Post{title: "1"})
+    user = TestRepo.insert!(%User{name: "1"})
+    TestRepo.insert!(%Comment{text: "1", post_id: post.id, author_id: user.id})
+    TestRepo.insert!(%Permalink{post_id: post.id, user_id: user.id})
+
+    query =
+      from c in Comment,
+        join: pp in assoc(c, :post_permalink),
+        join: ap in assoc(c, :author_permalink),
+        join: u1 in assoc(pp, :user),
+        join: u2 in assoc(ap, :user),
+        preload: [post_permalink: {pp, [:post, user: u1]}, author_permalink: {ap, [:post, user: u2]}]
+
+    [comment] = TestRepo.all(query)
+
+    assert not Ecto.assoc_loaded?(comment.post)
+    assert not Ecto.assoc_loaded?(comment.author)
+    assert %Permalink{user: %User{}, post: %Post{}} = comment.post_permalink
+    assert %Permalink{user: %User{}, post: %Post{}} = comment.author_permalink
+  end
+
+  test "joining nested through associations with a nested preloads" do
+    user = TestRepo.insert!(%User{name: "1"})
+    post = TestRepo.insert!(%Post{title: "1", author_id: user.id})
+    TestRepo.insert!(%Comment{text: "1", post_id: post.id})
+    TestRepo.insert!(%Permalink{post_id: post.id, user_id: user.id})
+
+    query =
+      from c in Comment,
+        join: pp in assoc(c, :post_permalink),
+        join: up in assoc(pp, :user_posts),
+        preload: [post_permalink: {pp, [:post, user_posts: {up, :comments}]}]
+
+    [comment] = TestRepo.all(query)
+
+    assert not Ecto.assoc_loaded?(comment.post)
+    assert %Permalink{post: %Post{}, user_posts: [%Post{}]} = comment.post_permalink
+    assert not Ecto.assoc_loaded?(comment.post_permalink.user)
+  end
 end

integration_test/support/schemas.exs

@@ -98,6 +98,7 @@ defmodule Ecto.Integration.Comment do
     belongs_to :post, Ecto.Integration.Post
     belongs_to :author, Ecto.Integration.User
     has_one :post_permalink, through: [:post, :permalink]
+    has_one :author_permalink, through: [:author, :permalink]
   end
 
   def changeset(schema, params) do
@@ -124,6 +125,7 @@ defmodule Ecto.Integration.Permalink do
     belongs_to :update_post, Ecto.Integration.Post, on_replace: :update, foreign_key: :post_id, define_field: false
     belongs_to :user, Ecto.Integration.User
     has_many :post_comments_authors, through: [:post, :comments_authors]
+    has_many :user_posts, through: [:user, :posts]
   end
 
   def changeset(schema, params) do

lib/ecto/repo/preloader.ex

@@ -12,14 +12,14 @@ defmodule Ecto.Repo.Preloader do
   Transforms a result set based on query preloads, loading
   the associations onto their parent schema.
   """
-  @spec query([list], Ecto.Repo.t, list, Access.t, fun, {adapter_meta :: map, opts :: Keyword.t}) :: [list]
-  def query([], _repo_name, _preloads, _take, _fun, _tuplet), do: []
-  def query(rows, _repo_name, [], _take, fun, _tuplet), do: Enum.map(rows, fun)
+  @spec query([list], Ecto.Repo.t, list, Access.t, list, fun, {adapter_meta :: map, opts :: Keyword.t}) :: [list]
+  def query([], _repo_name, _preloads, _take, _assocs, _fun, _tuplet), do: []
+  def query(rows, _repo_name, [], _take, _assocs, fun, _tuplet), do: Enum.map(rows, fun)
 
-  def query(rows, repo_name, preloads, take, fun, tuplet) do
+  def query(rows, repo_name, preloads, take, assocs, fun, tuplet) do
     rows
     |> extract()
-    |> normalize_and_preload_each(repo_name, preloads, take, tuplet)
+    |> normalize_and_preload_each(repo_name, preloads, take, assocs, tuplet)
     |> unextract(rows, fun)
   end
 
@@ -41,16 +41,17 @@ defmodule Ecto.Repo.Preloader do
   end
 
   def preload(structs, repo_name, preloads, {_adapter_meta, opts} = tuplet) when is_list(structs) do
-    normalize_and_preload_each(structs, repo_name, preloads, opts[:take], tuplet)
+    normalize_and_preload_each(structs, repo_name, preloads, opts[:take], [], tuplet)
   end
 
   def preload(struct, repo_name, preloads, {_adapter_meta, opts} = tuplet) when is_map(struct) do
-    normalize_and_preload_each([struct], repo_name, preloads, opts[:take], tuplet) |> hd()
+    normalize_and_preload_each([struct], repo_name, preloads, opts[:take], [], tuplet) |> hd()
   end
 
-  defp normalize_and_preload_each(structs, repo_name, preloads, take, tuplet) do
+  defp normalize_and_preload_each(structs, repo_name, preloads, take, query_assocs, tuplet) do
     preloads = normalize(preloads, take, preloads)
-    preload_each(structs, repo_name, preloads, tuplet)
+    query_assocs = normalize_query_assocs(query_assocs)
+    preload_each(structs, repo_name, preloads, query_assocs, tuplet)
   rescue
     e ->
       # Reraise errors so we ignore the preload inner stacktrace
@@ -59,21 +60,21 @@ defmodule Ecto.Repo.Preloader do
 
   ## Preloading
 
-  defp preload_each(structs, _repo_name, [], _tuplet),   do: structs
-  defp preload_each([], _repo_name, _preloads, _tuplet), do: []
-  defp preload_each(structs, repo_name, preloads, tuplet) do
+  defp preload_each(structs, _repo_name, [], _query_assocs, _tuplet),   do: structs
+  defp preload_each([], _repo_name, _preloads, _query_assocs, _tuplet), do: []
+  defp preload_each(structs, repo_name, preloads, query_assocs, tuplet) do
     if sample = Enum.find(structs, & &1) do
       module = sample.__struct__
       prefix = preload_prefix(tuplet, sample)
-      {assocs, throughs, embeds} = expand(module, preloads, {%{}, %{}, []})
+      {assocs, throughs, embeds} = expand(module, preloads, query_assocs, {%{}, [], []})
       structs = preload_embeds(structs, embeds, repo_name, tuplet)
+      structs = preload_throughs(structs, throughs, repo_name, query_assocs, tuplet)
 
       {fetched_assocs, to_fetch_queries} =
         prepare_queries(structs, module, assocs, prefix, repo_name, tuplet)
 
       fetched_queries = maybe_pmap(to_fetch_queries, repo_name, tuplet)
-      assocs = preload_assocs(fetched_assocs, fetched_queries, repo_name, tuplet)
-      throughs = Map.values(throughs)
+      assocs = preload_assocs(fetched_assocs, fetched_queries, repo_name, query_assocs, tuplet)
 
       for struct <- structs do
         struct = Enum.reduce assocs, struct, &load_assoc/2
@@ -148,23 +149,24 @@ defmodule Ecto.Repo.Preloader do
 
   # Then we unpack the query results, merge them, and preload recursively
   defp preload_assocs(
-         [{assoc, query?, loaded_ids, loaded_structs, preloads} | assocs],
+         [{assoc, query?, loaded_ids, loaded_structs, sub_preloads} | assocs],
          queries,
          repo_name,
+         query_assocs,
          tuplet
        ) do
     {fetch_ids, fetch_structs, queries} = maybe_unpack_query(query?, queries)
-    all = preload_each(Enum.reverse(loaded_structs, fetch_structs), repo_name, preloads, tuplet)
+    sub_query_assocs = Keyword.get(query_assocs, assoc.field, [])
+    all = preload_each(Enum.reverse(loaded_structs, fetch_structs), repo_name, sub_preloads, sub_query_assocs, tuplet)
     entry = {:assoc, assoc, assoc_map(assoc.cardinality, Enum.reverse(loaded_ids, fetch_ids), all)}
-    [entry | preload_assocs(assocs, queries, repo_name, tuplet)]
+    [entry | preload_assocs(assocs, queries, repo_name, query_assocs, tuplet)]
   end
 
-  defp preload_assocs([], [], _repo_name, _tuplet), do: []
+  defp preload_assocs([] = _assocs, [] = _queries, _, _, _), do: []
 
-  defp preload_embeds(structs, [], _repo_name, _tuplet), do: structs
+  defp preload_embeds(structs, [] = _embeds, _, _), do: structs
 
   defp preload_embeds(structs, [embed | embeds], repo_name, tuplet) do
-
     {%{field: field, cardinality: card}, sub_preloads} = embed
 
     {embed_structs, counts} =
@@ -176,23 +178,57 @@ defmodule Ecto.Repo.Preloader do
         struct, _counts -> raise ArgumentError, "expected #{inspect(struct)} to contain embed `#{field}`"
       end)
 
-    embed_structs = preload_each(embed_structs, repo_name, sub_preloads, tuplet)
-    structs = load_embeds(card, field, structs, embed_structs, Enum.reverse(counts), [])
+    # It is not possible for an embed to preload through a join
+    # Therefore, we don't consider associations coming from queries
+    embed_structs = preload_each(embed_structs, repo_name, sub_preloads, [], tuplet)
+    structs = put_through_or_embed(card, field, structs, embed_structs, Enum.reverse(counts), [])
     preload_embeds(structs, embeds, repo_name, tuplet)
   end
 
-  defp load_embeds(_card, _field, [], [], [], acc), do: Enum.reverse(acc)
+  defp preload_throughs(structs, [] = _throughs, _, _, _), do: structs
 
-  defp load_embeds(card, field, [struct | structs], embed_structs, [0 | counts], acc),
-    do: load_embeds(card, field, structs, embed_structs, counts, [struct | acc])
+  defp preload_throughs(
+         structs,
+         [{_, _, false = _from_query?} | throughs],
+         repo_name,
+         query_assocs,
+         tuplet
+       ) do
+    # We only preload through associations directly if they were previously loaded by a query.
+    # Otherwise, we preload the association chain.
+    preload_throughs(structs, throughs, repo_name, query_assocs, tuplet)
+  end
 
-  defp load_embeds(:one, field, [struct | structs], [embed_struct | embed_structs], [1 | counts], acc),
-    do: load_embeds(:one, field, structs, embed_structs, counts, [Map.put(struct, field, embed_struct) | acc])
+  defp preload_throughs(structs, [through | throughs], repo_name, query_assocs, tuplet) do
+    {{_, %{field: field, cardinality: card}, _}, sub_preloads, true} = through
+    sub_query_assocs = Keyword.get(query_assocs, field, [])
 
-  defp load_embeds(:many, field, [struct | structs], embed_structs, [count | counts], acc) do
-    {current_embeds, rest_embeds} = split_n(embed_structs, count, [])
-    acc = [Map.put(struct, field, Enum.reverse(current_embeds)) | acc]
-    load_embeds(:many, field, structs, rest_embeds, counts, acc)
+    {through_structs, counts} =
+      Enum.flat_map_reduce(structs, [], fn
+        %{^field => throughs}, counts when is_list(throughs) -> {throughs, [length(throughs) | counts]}
+        %{^field => nil}, counts -> {[], [0 | counts]}
+        %{^field => through}, counts -> {[through], [1 | counts]}
+        nil, counts -> {[], [0 | counts]}
+        struct, _counts -> raise ArgumentError, "expected #{inspect(struct)} to contain through association `#{field}`"
+      end)
+
+    through_structs = preload_each(through_structs, repo_name, sub_preloads, sub_query_assocs, tuplet)
+    structs = put_through_or_embed(card, field, structs, through_structs, Enum.reverse(counts), [])
+    preload_throughs(structs, throughs, repo_name, query_assocs, tuplet)
+  end
+
+  defp put_through_or_embed(_card, _field, [], [], [], acc), do: Enum.reverse(acc)
+
+  defp put_through_or_embed(card, field, [struct | structs], loaded_structs, [0 | counts], acc),
+    do: put_through_or_embed(card, field, structs, loaded_structs, counts, [struct | acc])
+
+  defp put_through_or_embed(:one, field, [struct | structs], [loaded | loaded_structs], [1 | counts], acc),
+    do: put_through_or_embed(:one, field, structs, loaded_structs, counts, [Map.put(struct, field, loaded) | acc])
+
+  defp put_through_or_embed(:many, field, [struct | structs], loaded_structs, [count | counts], acc) do
+    {current_loaded, rest_loaded} = split_n(loaded_structs, count, [])
+    acc = [Map.put(struct, field, Enum.reverse(current_loaded)) | acc]
+    put_through_or_embed(:many, field, structs, rest_loaded, counts, acc)
   end
 
   defp maybe_unpack_query(false, queries), do: {[], [], queries}
@@ -440,14 +476,19 @@ defmodule Ecto.Repo.Preloader do
     Map.put(struct, field, loaded)
   end
 
-  defp load_through({:through, _assoc, _throughs}, nil) do
+  defp load_through({_, _, _}, nil) do
     nil
   end
 
-  defp load_through({:through, assoc, throughs}, struct) do
+  defp load_through({{:through, assoc, throughs}, _, from_query?}, struct) do
     %{cardinality: cardinality, field: field, owner: owner} = assoc
-    {loaded, _} = Enum.reduce(throughs, {[struct], owner}, &recur_through/2)
-    Map.put(struct, field, maybe_first(loaded, cardinality))
+
+    if from_query? do
+      struct
+    else
+      {loaded, _} = Enum.reduce(throughs, {[struct], owner}, &recur_through/2)
+      Map.put(struct, field, maybe_first(loaded, cardinality))
+    end
   end
 
   defp maybe_first(list, :one), do: List.first(list)
@@ -574,9 +615,19 @@ defmodule Ecto.Repo.Preloader do
                          "preload expects an atom, a (nested) keyword or a (nested) list of atoms"
   end
 
+  defp normalize_query_assocs([]), do: []
+
+  defp normalize_query_assocs(assocs) when is_list(assocs) do
+    Enum.reduce(assocs, [], &normalize_each_query_assoc(&1, &2))
+  end
+
+  defp normalize_each_query_assoc({field, {_idx, sub_assocs}}, acc) do
+    [{field, normalize_query_assocs(sub_assocs)} | acc]
+  end
+
   ## Expand
 
-  def expand(schema, preloads, acc) do
+  def expand(schema, preloads, query_assocs, acc) do
     Enum.reduce(preloads, acc, fn {preload, {fields, query, sub_preloads}},
                                   {assocs, throughs, embeds} ->
       assoc_or_embed = association_or_embed!(schema, preload)
@@ -590,13 +641,17 @@ defmodule Ecto.Repo.Preloader do
           {assocs, throughs, embeds}
 
         {:through, _, through} ->
-          through =
-            through
-            |> Enum.reverse()
-            |> Enum.reduce({fields, query, sub_preloads}, &{nil, nil, [{&1, &2}]})
-            |> elem(2)
+          if Keyword.has_key?(query_assocs, preload) do
+            {assocs, [{info, sub_preloads, true} | throughs], embeds}
+          else
+            through =
+              through
+              |> Enum.reverse()
+              |> Enum.reduce({fields, query, sub_preloads}, &{nil, nil, [{&1, &2}]})
+              |> elem(2)
 
-          expand(schema, through, {assocs, Map.put(throughs, preload, info), embeds})
+            expand(schema, through, query_assocs, {assocs, [{info, sub_preloads, false} | throughs], embeds})
+          end
 
         :embed ->
           if sub_preloads == [] do

lib/ecto/repo/queryable.ex

@@ -235,7 +235,7 @@ defmodule Ecto.Repo.Queryable do
         {count,
          rows
          |> Ecto.Repo.Assoc.query(assocs, sources, preprocessor)
-         |> Ecto.Repo.Preloader.query(name, preloads, take, postprocessor, tuplet)}
+         |> Ecto.Repo.Preloader.query(name, preloads, take, assocs, postprocessor, tuplet)}
     end
   end