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fix selecting list of fields and map/2 from subquery #4777

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Merged
greg-rychlewski merged 8 commits into from
Aug 5, 2026
+26 −8
Merged

fix selecting list of fields and map/2 from subquery #4777

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34ad651
fix selecting list of fields and map/2 from subquery
greg-rychlewski Aug 4, 2026
9dcb959
fixes
greg-rychlewski Aug 4, 2026
895acce
fixes
greg-rychlewski Aug 4, 2026
1e57301
fixes
greg-rychlewski Aug 4, 2026
6f33969
fixes
greg-rychlewski Aug 4, 2026
e69d357
update test
greg-rychlewski Aug 4, 2026
766e236
fixes
greg-rychlewski Aug 4, 2026
23d8c59
fixes
greg-rychlewski Aug 4, 2026
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21 changes: 13 additions & 8 deletions lib/ecto/query/planner.ex
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Original file line number Diff line number Diff line change
Expand Up @@ -2258,8 +2258,8 @@ defmodule Ecto.Query.Planner do
{{:ok, {:struct, _}}, {:fragment, _, _}} ->
error!(query, "it is not possible to return a struct subset of a fragment")

{{:ok, {:struct, fields}}, %Ecto.SubQuery{select: select}} ->
subquery_select_fields(select, fields, ix, query)
{{:ok, {kind, fields}}, %Ecto.SubQuery{select: select}} ->
subquery_select_fields(kind, select, fields, ix, query)

{{:ok, {_, []}}, {_, _, _}} ->
error!(
Expand Down Expand Up @@ -2336,16 +2336,21 @@ defmodule Ecto.Query.Planner do
end)
end

defp subquery_select_fields(select, requested_fields, ix, query) do
defp subquery_select_fields(kind, select, requested_fields, ix, query) do
kind = if kind == :any, do: :struct, else: kind
available_fields = subquery_source_fields(select)
requested_fields = List.wrap(requested_fields)

schema =
case select do
{:source, {_, schema}, _, _} when not is_nil(schema) -> schema
case {kind, select} do
{:struct, {:source, {_, schema}, _, _}} when not is_nil(schema) ->
schema

_ ->
error!(query, "it is not possible to return a struct subset of a subquery that does not return a schema struct")
{:map, _} ->
nil

{:struct, _} ->
error!(query, "it is not possible to return a struct subset of a subquery that does not return a schema struct")
end

types =
Expand All @@ -2355,7 +2360,7 @@ defmodule Ecto.Query.Planner do
{field, type}

:error ->
error!(query, "field `#{field}` in struct/2 is not available in the subquery. " <>
error!(query, "field `#{field}` is not available in the subquery. " <>
"Subquery only returns fields: #{inspect(available_fields)}")
end
end)
Expand Down
14 changes: 14 additions & 0 deletions test/ecto/query/planner_test.exs
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Expand Up @@ -2670,6 +2670,20 @@ defmodule Ecto.Query.PlannerTest do
] = query.select.fields
end

test "normalze: select list of fields from subquery source" do
{_, _, _, select} = subquery(Post) |> select([p], [:title]) |> normalize_with_params()
%{from: {_, {:source, {_, postprocess_schema}, _, types}}} = select
assert postprocess_schema == Post
assert types == [title: :string]
end

test "normalze: select map/2 from subquery source" do
{_, _, _, select} = subquery(Post) |> select([p], map(p, [:title])) |> normalize_with_params()
%{from: {_, {:source, {_, postprocess_schema}, _, types}}} = select
assert postprocess_schema == nil
assert types == [title: :string]
end

test "normalize: select with :%{}" do
query = Post |> select([p], %{p | title: "foo"}) |> normalize()
assert query.select.expr == {:%{}, [], [{:|, [], [{:&, [], [0]}, [title: "foo"]]}]}
Expand Down
2 changes: 1 addition & 1 deletion test/ecto/query/subquery_test.exs
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Expand Up @@ -440,7 +440,7 @@ defmodule Ecto.Query.SubqueryTest do
query = normalize(from(c in Comment, join: p in subquery(subquery), on: true, select: p.title))
assert query.select.fields == [{{:., [type: :string], [{:&, [], [1]}, :title]}, [], []}]

subquery = from p in Post, select: %{id: p.id, title: p.title}
subquery = from p in Post, select: struct(p, [:id, :title])

@greg-rychlewski greg-rychlewski Aug 4, 2026

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@josevalim The fix was surfacing an error in this test. Basically it fails now because select: [atom()] is considered the same as struct/2 more strictly.

Just want to make sure this is still the right interpretation of the list of atoms. It's in the docs but not sure if it's out of date

It is also possible to select a struct and limit the returned fields at the same time:

from(City, select: [:name])

The syntax above is equivalent to:

from(city in City, select: struct(city, [:name]))

@josevalim josevalim Aug 4, 2026

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Yeah, select: [:foo, :bar] keeps whatever the from clause is. Structs if there are source+schema, maps if we only have the source.

@greg-rychlewski greg-rychlewski Aug 4, 2026

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ah ok I believe what I just pushed now is the correct solution then. sorry for the back and forth. would you mind taking a look once more?

query = normalize(from(p in subquery(subquery), select: [:title]))
assert [{{:., _, [{:&, [], [0]}, :title]}, [], []}] = query.select.fields

Expand Down
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