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Load rows for limit/offset queries with lateral join
Queries that do not rely on limit/offset continue to use the existing approach, where a simple where clause is added to the query. Complex queries are converted into a lateral subquery. The outer query contains the inputs. Ideally the query would use `unnest` or a similar function to efficiently load an array of values into a query, but Ecto doesn't offer such functionality in a portable way. The workaround here is to run a distinct query that selects the column from the queryable where the value is any of the inputs.
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,64 @@ | ||
| defmodule Dataloader.LimitQueryTest do | ||
| use ExUnit.Case, async: true | ||
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| alias Dataloader.{User, Post} | ||
| import Ecto.Query | ||
| alias Dataloader.TestRepo, as: Repo | ||
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| setup do | ||
| :ok = Ecto.Adapters.SQL.Sandbox.checkout(Repo) | ||
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| test_pid = self() | ||
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| source = | ||
| Dataloader.Ecto.new( | ||
| Repo, | ||
| query: &query(&1, &2, test_pid) | ||
| ) | ||
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| loader = | ||
| Dataloader.new() | ||
| |> Dataloader.add_source(Test, source) | ||
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| {:ok, loader: loader} | ||
| end | ||
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| defp query(Post, %{limit: limit}, test_pid) do | ||
| send(test_pid, :querying) | ||
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| Post | ||
| |> where([p], is_nil(p.deleted_at)) | ||
| |> order_by(asc: :id) | ||
| |> limit(^limit) | ||
| end | ||
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| defp query(queryable, _args, test_pid) do | ||
| send(test_pid, :querying) | ||
| queryable | ||
| end | ||
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| test "Query limit does not apply globally", %{loader: loader} do | ||
| user1 = %User{username: "Ben Wilson"} |> Repo.insert!() | ||
| user2 = %User{username: "Bruce Williams"} |> Repo.insert!() | ||
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| [post1, _post2, post3, _post4] = | ||
| [ | ||
| %Post{user_id: user1.id, title: "foo"}, | ||
| %Post{user_id: user1.id, title: "baz"}, | ||
| %Post{user_id: user2.id, title: "bar"}, | ||
| %Post{user_id: user2.id, title: "qux"} | ||
| ] | ||
| |> Enum.map(&Repo.insert!/1) | ||
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| args = {{:many, Post}, %{limit: 1}} | ||
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| loader = | ||
| loader | ||
| |> Dataloader.load(Test, args, user_id: user1.id) | ||
| |> Dataloader.load(Test, args, user_id: user2.id) | ||
| |> Dataloader.run() | ||
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| assert [post1] == Dataloader.get(loader, Test, args, user_id: user1.id) | ||
| assert [post3] == Dataloader.get(loader, Test, args, user_id: user2.id) | ||
| end | ||
| end |