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greg-rychlewski merged 3 commits into from
Nov 8, 2025
+52 −3
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support selecting a subset of a subquery as a struct #4678

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f2817c0
support selecting a subset of a subquery as a struct
zachdaniel Nov 8, 2025
b6d7b9b
don't do double traversal, don't include type information, remove unt…
zachdaniel Nov 8, 2025
54b3ceb
use better function name for subquery fields
zachdaniel Nov 8, 2025
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33 changes: 31 additions & 2 deletions lib/ecto/query/planner.ex
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Original file line number Diff line number Diff line change
Expand Up @@ -2207,8 +2207,8 @@ defmodule Ecto.Query.Planner do
{{:ok, {:struct, _}}, {:fragment, _, _}} ->
error!(query, "it is not possible to return a struct subset of a fragment")

{{:ok, {:struct, _}}, %Ecto.SubQuery{}} ->
error!(query, "it is not possible to return a struct subset of a subquery")
{{:ok, {:struct, fields}}, %Ecto.SubQuery{select: select}} ->
subquery_select_fields(select, fields, ix, query)

{{:ok, {_, []}}, {_, _, _}} ->
error!(
Expand Down Expand Up @@ -2280,6 +2280,35 @@ defmodule Ecto.Query.Planner do
end)
end

defp subquery_select_fields(select, requested_fields, ix, query) do
available_fields = subquery_source_fields(select)
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@greg-rychlewski greg-rychlewski Nov 8, 2025

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I'm not able to check atm but I believe error! prints the query so it might be unnecessary to include this in the message

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@zachdaniel zachdaniel Nov 8, 2025

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🤔 I don't think I'm currently adding the query to any error message, but maybe I'm missing it. Could you point out the line?

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@greg-rychlewski greg-rychlewski Nov 8, 2025

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oh sorry I meant that available_fields is only used here:

error!(query, "field `#{field}` in struct/2 is not available in the subquery. " <>
                         "Subquery only returns fields: #{inspect(available_fields)}")

and i think it might be unnecessary because the error! function already prints the entire query. it raises Ecto.QueryError which takes care of it. so you are already told which fields exist in that way.

it's not a big deal either way it was just a tiny cleanup (if my assumptions are right)

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@zachdaniel zachdaniel Nov 8, 2025

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Ah, gotcha. I think I'd prefer this because when the query gets big (like in my current case) or involves multiple subqueries for example, it's not always entirely obvious. Listing the available fields I think will make it much clearer.

requested_fields = List.wrap(requested_fields)

schema =
case select do
{: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")
end

types =
Enum.map(requested_fields, fn field ->
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case subquery_type_for(select, field) do
{:ok, type} ->
{field, type}

:error ->
error!(query, "field `#{field}` in struct/2 is not available in the subquery. " <>
"Subquery only returns fields: #{inspect(available_fields)}")
end
end)

field_exprs = Enum.map(requested_fields, &select_field(&1, ix, :always))

{{:source, {nil, schema}, nil, types}, field_exprs}
end

defp select_field(field, ix, writable) do
{{:., [writable: writable], [{:&, [], [ix]}, field]}, [], []}
end
Expand Down
22 changes: 21 additions & 1 deletion test/ecto/query/subquery_test.exs
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Expand Up @@ -447,8 +447,28 @@ defmodule Ecto.Query.SubqueryTest do
subquery = from p in Post, select: %{id: p.id, title: p.title}
query = normalize(from(p in subquery(subquery), select: map(p, [:title])))
assert [{{:., _, [{:&, [], [0]}, :title]}, [], []}] = query.select.fields
end

test "struct/2 with subqueries" do
subquery = from p in Post, select: p
query = normalize(from(p in subquery(subquery), select: struct(p, [:id, :title])))
assert query.select.fields == [
{{:., [writable: :always], [{:&, [], [0]}, :id]}, [], []},
{{:., [writable: :always], [{:&, [], [0]}, :title]}, [], []}
]

subquery = from p in Post, select: p
query = normalize(from(c in Comment, join: p in subquery(subquery), on: true, select: struct(p, [:title])))
assert query.select.fields == [{{:., [writable: :always], [{:&, [], [1]}, :title]}, [], []}]

subquery = from p in Post, select: struct(p, [:id, :title, :text])
query = normalize(from(p in subquery(subquery), select: struct(p, [:id, :title])))
assert query.select.fields == [
{{:., [writable: :always], [{:&, [], [0]}, :id]}, [], []},
{{:., [writable: :always], [{:&, [], [0]}, :title]}, [], []}
]

assert_raise Ecto.QueryError, ~r/it is not possible to return a struct subset of a subquery/, fn ->
assert_raise Ecto.QueryError, ~r/it is not possible to return a struct subset of a subquery that does not return a schema struct/, fn ->
subquery = from p in Post, select: %{id: p.id, title: p.title}
normalize(from(p in subquery(subquery), select: struct(p, [:title])))
end
Expand Down
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