Add Ecto.Multi.exists?/4 (#4063)

lib/ecto/multi.ex

@@ -414,7 +414,7 @@ defmodule Ecto.Multi do
   @spec one(
           t,
           name,
-          queryable :: Ecto.Queryable.t | (any -> Ecto.Queryable.t),
+          queryable :: Ecto.Queryable.t | fun(Ecto.Queryable.t),
           opts :: Keyword.t
         ) :: t
   def one(multi, name, queryable_or_fun, opts \\ [])
@@ -445,7 +445,7 @@ defmodule Ecto.Multi do
   @spec all(
           t,
           name,
-          queryable :: Ecto.Queryable.t | (any -> Ecto.Queryable.t),
+          queryable :: Ecto.Queryable.t | fun(Ecto.Queryable.t),
           opts :: Keyword.t
         ) :: t
   def all(multi, name, queryable_or_fun, opts \\ [])
@@ -458,6 +458,37 @@ defmodule Ecto.Multi do
     run(multi, name, operation_fun({:all, fn _ -> queryable end}, opts))
   end
 
+  @doc """
+  Checks if there exists an entry matching the given query and stores a boolean in the multi.
+
+  Accepts the same arguments and options as `c:Ecto.Repo.exists?/2`.
+
+  ## Example
+
+      Ecto.Multi.new()
+      |> Ecto.Multi.exists?(:post, Post)
+      |> MyApp.Repo.transaction()
+
+      Ecto.Multi.new()
+      |> Ecto.Multi.exists?(:post, fn _changes -> Post end)
+      |> MyApp.Repo.transaction()
+  """
+  @spec exists?(
+          t,
+          name,
+          queryable :: Ecto.Queryable.t | fun(Ecto.Queryable.t),
+          opts :: Keyword.t
+        ) :: t
+  def exists?(multi, name, queryable_or_fun, opts \\ [])
+
+  def exists?(multi, name, fun, opts) when is_function(fun, 1) do
+    run(multi, name, operation_fun({:exists?, fun}, opts))
+  end
+
+  def exists?(multi, name, queryable, opts) do
+    run(multi, name, operation_fun({:exists?, fn _ -> queryable end}, opts))
+  end
+
   defp add_changeset(multi, action, name, changeset, opts) when is_list(opts) do
     add_operation(multi, name, {:changeset, put_action(changeset, action), opts})
   end
@@ -870,6 +901,12 @@ defmodule Ecto.Multi do
     end
   end
 
+  defp operation_fun({:exists?, fun}, opts) do
+    fn repo, changes ->
+      {:ok, repo.exists?(fun.(changes), opts)}
+    end
+  end
+
   defp operation_fun({operation, fun}, opts) do
     fn repo, changes ->
       apply(repo, operation, [fun.(changes), opts])

test/ecto/multi_test.exs

@@ -190,6 +190,26 @@ defmodule Ecto.MultiTest do
     assert [{:comments, {:run, _fun}}] = multi.operations
   end
 
+  test "exists? queryable" do
+    multi =
+      Multi.new()
+      |> Multi.exists?(:comment, Comment)
+
+    assert multi.names == MapSet.new([:comment])
+    assert [{:comment, {:run, _fun}}] = multi.operations
+  end
+
+  test "exists? fun" do
+    fun = fn _changes -> Comment end
+
+    multi =
+      Multi.new()
+      |> Multi.exists?(:comment, fun)
+
+    assert multi.names == MapSet.new([:comment])
+    assert [{:comment, {:run, _fun}}] = multi.operations
+  end
+
   test "error" do
     multi =
       Multi.new()