support max_total_entries option

lib/scrivener/paginater/ecto/query.ex

@@ -67,13 +67,30 @@ defimpl Scrivener.Paginater, for: Ecto.Query do
     |> exclude(:order_by)
     |> exclude(:select)
     |> select([{x, source_index}], struct(x, ^[field]))
+    |> maybe_limit_by_max_total_entries(options)
     |> subquery()
     |> select(count("*"))
     |> repo.one(options)
   end
 
   defp aggregate(query, repo, options) do
-    repo.aggregate(query, :count, options)
+    query
+    |> maybe_limit_by_max_total_entries(options)
+    |> repo.aggregate(:count, options)
+  end
+
+  defp maybe_limit_by_max_total_entries(query, options) do
+    case Keyword.get(options, :max_total_entries) do
+      nil ->
+        query
+
+      max_total_entries when is_integer(max_total_entries) and max_total_entries > 0 ->
+        limit(query, ^max_total_entries)
+
+      otherwise ->
+        raise ArgumentError,
+              "max_total_entries must be a positive integer, got: #{inspect(otherwise)}"
+    end
   end
 
   defp total_pages(0, _), do: 1

test/scrivener/paginator/ecto/query_test.exs

@@ -233,6 +233,32 @@ defmodule Scrivener.Paginator.Ecto.QueryTest do
       assert page.total_entries == 130
     end
 
+    test "will respect max_total_entries passed to paginate" do
+      create_posts()
+
+      page =
+        Post
+        |> Post.published()
+        |> Scrivener.Ecto.Repo.paginate(options: [max_total_entries: 4])
+
+      assert length(page.entries) == 5
+      assert page.total_entries == 4
+    end
+
+    test "will respect max_total_entries passed to paginate with a group by clause on field on joined table" do
+      create_posts()
+
+      page =
+        Post
+        |> join(:inner, [p], c in assoc(p, :comments))
+        |> group_by([p, c], c.body)
+        |> select([p, c], {c.body, count("*")})
+        |> Scrivener.Ecto.Repo.paginate(options: [max_total_entries: 1])
+
+      assert length(page.entries) == 2
+      assert page.total_entries == 1
+    end
+
     test "will use total_pages if page_numer is too large" do
       posts = create_posts()