PostgreSQL does not support selecting all fields from without a schema

sean

TL;DR The problem I’m trying to solve is analogous to building an Ecto.Query, say, for posts that includes n number of latest associated comments for each.

I realize this has been discussed several times already, but I still have not found a clear solution:

As an example, given the following:

defmodule Comment do
  schema "comments" do
    belongs_to(:post, Post)
  end
end

defmodule Post do
  schema "posts" do
    has_many(:comments, Comment)    
  end

  defmacrop latest_comments(post_id, limit \\ 5) do
    quote do
      fragment(
        "select * from comments
        where comments.post_id = ?
        order by comments.inserted_at desc nulls last
        limit ?",
        unquote(post_id),
        unquote(limit)
      )
    end
  end

  def with_latest_comments(queryable) do
    queryable
    |> join(:left_lateral, [p], c in latest_comments(p.id))
    |> select_merge([p, c], %{comments: c})
  end
end

I think this is so close to a solution, but it raises this error:

PostgreSQL does not support selecting all fields from without a schema. Please specify a schema or specify exactly which fields you want to select in query

If I wrap c with map or struct e.g.:

|> select_merge([p, c], %{comments: struct(c, [:id, :post_id])})

The following error is raised:

it is not possible to return a map/struct subset of a fragment, you must explicitly return the desired individual fields in query

I see this was discussed here #1592 Ecto.QueryError) PostgreSQL requires a schema module when using selector “u0” but none was given.

The following suggestions were mentioned, either of which I think would work great!

We could also support struct(u, [:foo, :bar, :baz], MyModule)
…
or even struct(u, MyModule) if there are all the columns since the module contains the column names

But alas, that issue is from 2016 and has long been closed.

Any other ideas? Am I missing something obvious?

Thanks!

2 comments

/phoenix #ecto #sql #postgres #ecto-query

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2018-06-29 18:11:27 UTC

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peerreynders

Edit: Full example of a working lateral join.

iex(1)> alias MusicDB.{Repo,Album,Track}
[MusicDB.Repo, MusicDB.Album, MusicDB.Track]
iex(2)> import Ecto.Query
Ecto.Query
iex(3)> compact_rest = fn
...(3)>   {nil,nil,nil,[]} ->
...(3)>     []
...(3)>   {_, album, tracks, rest} = init ->
...(3)>     [Map.put(album, :tracks, :lists.reverse(tracks)) | rest]
...(3)> end
#Function<6.127694169/1 in :erl_eval.expr/5>
iex(4)> next_acc = fn
...(4)>   (%{id: id} = a, t, {id, album, tracks, rest}) ->
...(4)>     {id, album, [t|tracks], rest}
...(4)>   (%{id: id} = a, t, acc) ->
...(4)>     {id, a, [t], compact_rest.(acc)}
...(4)> end
#Function<18.127694169/3 in :erl_eval.expr/5>
iex(5)> row_to_acc = fn (%{album: album, track: track}, acc) -> next_acc.(album, track, acc) end
#Function<12.127694169/2 in :erl_eval.expr/5>
iex(6)> make_result = fn rows ->
...(6)>   rows
...(6)>   |> List.foldl({nil,nil,nil,[]}, row_to_acc)
...(6)>   |> compact_rest.()
...(6)>   |> :lists.reverse()
...(6)> end
#Function<6.127694169/1 in :erl_eval.expr/5>
iex(7)> album_ids = [1,2]
[1, 2]
iex(8)> album_query = from(a in Album, [where: a.id in ^album_ids])
#Ecto.Query<from a in MusicDB.Album, where: a.id in ^[1, 2]>
iex(9)> query = from(q in subquery(album_query), [
...(9)>   left_lateral_join: t in fragment("SELECT * FROM tracks AS t WHERE t.album_id = ? ORDER BY t.inserted_at DESC NULLS LAST LIMIT ?", q.id,3), on: true,
...(9)>   select: %{album: %{id: q.id, artist_id: q.artist_id, title: q.title}},
...(9)>   select_merge: %{track: %{id: t.id, index: t.index, title: t.title, duration: t.duration}}
...(9)> ])
#Ecto.Query<from a in subquery(from a in MusicDB.Album,
  where: a.id in ^[1, 2]),
 left_join_lateral: f in fragment("SELECT * FROM tracks AS t WHERE t.album_id = ? ORDER BY t.inserted_at DESC NULLS LAST LIMIT ?", a.id, 3),
 on: true,
 select: %{album: %{id: a.id, artist_id: a.artist_id, title: a.title}, track: %{id: f.id, index: f.index, title: f.title, duration: f.duration}}>
iex(10)> results = Repo.all(query)

18:31:36.947 [debug] QUERY OK db=2.5ms
SELECT s0."id", s0."artist_id", s0."title", f1."id", f1."index", f1."title", f1."duration" FROM (SELECT a0."id" AS "id", a0."title" AS "title", a0."inserted_at" AS "inserted_at", a0."updated_at" AS "updated_at", a0."artist_id" AS "artist_id" FROM "albums" AS a0 WHERE (a0."id" = ANY($1))) AS s0 LEFT OUTER JOIN LATERAL (SELECT * FROM tracks AS t WHERE t.album_id = s0."id" ORDER BY t.inserted_at DESC NULLS LAST LIMIT 3) AS f1 ON TRUE [[1, 2]]
[
  %{
    album: %{artist_id: 1, id: 1, title: "Kind Of Blue"},
    track: %{duration: 481, id: 5, index: 5, title: "Flamenco Sketches"}
  },
  %{
    album: %{artist_id: 1, id: 1, title: "Kind Of Blue"},
    track: %{duration: 693, id: 4, index: 4, title: "All Blues"}
  },
  %{
    album: %{artist_id: 1, id: 1, title: "Kind Of Blue"},
    track: %{duration: 327, id: 3, index: 3, title: "Blue In Green"}
  },
  %{
    album: %{artist_id: 1, id: 2, title: "Cookin' At The Plugged Nickel"},
    track: %{duration: 1061, id: 10, index: 5, title: "No Blues"}
  },
  %{
    album: %{artist_id: 1, id: 2, title: "Cookin' At The Plugged Nickel"},
    track: %{duration: 754, id: 9, index: 4, title: "Miles"}
  },
  %{
    album: %{artist_id: 1, id: 2, title: "Cookin' At The Plugged Nickel"},
    track: %{duration: 896, id: 8, index: 3, title: "Walkin'"}
  }
]
iex(11)> result = make_result.(results)
[
  %{
    artist_id: 1,
    id: 1,
    title: "Kind Of Blue",
    tracks: [
      %{duration: 481, id: 5, index: 5, title: "Flamenco Sketches"},
      %{duration: 693, id: 4, index: 4, title: "All Blues"},
      %{duration: 327, id: 3, index: 3, title: "Blue In Green"}
    ]
  },
  %{ 
    artist_id: 1,
    id: 2,
    title: "Cookin' At The Plugged Nickel",
    tracks: [
      %{duration: 1061, id: 10, index: 5, title: "No Blues"},
      %{duration: 754, id: 9, index: 4, title: "Miles"},
      %{duration: 896, id: 8, index: 3, title: "Walkin'"}
    ]
  }
]
iex(12)>

Raw SQL that served as the template:

SELECT a1.id AS album_id, a1.artist_id, a1.title AS album_title, t1.id AS track_id, t1.index, t1.title, t1.duration FROM
  (SELECT id, artist_id, title FROM albums WHERE id in (1,2)) a1
  LEFT JOIN LATERAL
  (SELECT id, index, title, duration FROM tracks WHERE album_id = a1.id ORDER BY inserted_at DESC NULLS LAST LIMIT 3) t1
  ON true;

Post #1

peerreynders

Correction: Caveat

The issue is that the conditions apply to all the records returned by that single preload query - not just each single association.

preload query.

Example:

iex(1)> alias MusicDB.{Repo,Album,Track}
[MusicDB.Repo, MusicDB.Album, MusicDB.Track]
iex(2)> import Ecto.Query
Ecto.Query
iex(3)> album_id = 2
2
iex(4)> tracks_query = from(t in Track, [
...(4)>    order_by: fragment("? DESC NULLS LAST", t.inserted_at),
...(4)>    limit: 3
...(4)> ])
#Ecto.Query<from t in MusicDB.Track,
 order_by: [asc: fragment("? DESC NULLS LAST", t.inserted_at)], limit: 3>
iex(5)> query = from(a in Album, [
...(5)>   preload: [tracks: ^tracks_query],
...(5)>   where: a.id == ^album_id
...(5)> ])
#Ecto.Query<from a in MusicDB.Album, where: a.id == ^2,
 preload: [tracks: #Ecto.Query<from t in MusicDB.Track, order_by: [asc: fragment("? DESC NULLS LAST", t.inserted_at)], limit: 3>]>
iex(6)> Repo.all(query)

12:43:52.217 [debug] QUERY OK source="albums" db=2.2ms decode=1.7ms
SELECT a0."id", a0."title", a0."inserted_at", a0."updated_at", a0."artist_id" FROM "albums" AS a0 WHERE (a0."id" = $1) [2]
 
12:43:52.223 [debug] QUERY OK source="tracks" db=2.5ms
SELECT t0."id", t0."title", t0."duration", t0."index", t0."number_of_plays", t0."inserted_at", t0."updated_at", t0."album_id", t0."album_id" FROM "tracks" AS t0 WHERE (t0."album_id" = $1) ORDER BY t0."album_id", t0."inserted_at" DESC NULLS LAST LIMIT 3 [2]
[
  %MusicDB.Album{
    __meta__: #Ecto.Schema.Metadata<:loaded, "albums">,
    artist: #Ecto.Association.NotLoaded<association :artist is not loaded>,
    artist_id: 1,
    genres: #Ecto.Association.NotLoaded<association :genres is not loaded>,
    id: 2,
    inserted_at: ~N[2018-06-16 20:29:41.476625],
    title: "Cookin' At The Plugged Nickel",
    tracks: [
      %MusicDB.Track{
        __meta__: #Ecto.Schema.Metadata<:loaded, "tracks">,
        album: #Ecto.Association.NotLoaded<association :album is not loaded>,
        album_id: 2,
        duration: 1061,
        duration_string: nil,
        id: 10,
        index: 5,
        inserted_at: ~N[2018-06-16 20:29:41.480612],
        number_of_plays: 0,
        title: "No Blues",
        updated_at: ~N[2018-06-16 20:29:41.480618]
      },
      %MusicDB.Track{
        __meta__: #Ecto.Schema.Metadata<:loaded, "tracks">,
        album: #Ecto.Association.NotLoaded<association :album is not loaded>,
        album_id: 2,
        duration: 754,
        duration_string: nil,
        id: 9,
        index: 4,
        inserted_at: ~N[2018-06-16 20:29:41.480045],
        number_of_plays: 0,
        title: "Miles",
        updated_at: ~N[2018-06-16 20:29:41.480051]
      },
      %MusicDB.Track{
        __meta__: #Ecto.Schema.Metadata<:loaded, "tracks">,
        album: #Ecto.Association.NotLoaded<association :album is not loaded>,
        album_id: 2,
        duration: 896,
        duration_string: nil,
        id: 8,
        index: 3,
        inserted_at: ~N[2018-06-16 20:29:41.479460],
        number_of_plays: 0,
        title: "Walkin'",
        updated_at: ~N[2018-06-16 20:29:41.479465]
      }
    ],
    updated_at: ~N[2018-06-16 20:29:41.476631]
  }
]
iex(7)>

Post #2

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