fix(update/4): improve Mongo.update/4 function

- prevent endless recursion when supplied empty list
- make options list optional
- add more documentation + tests

lib/mongo.ex

@@ -1129,7 +1129,29 @@ defmodule Mongo do
 
   e.g. long-hand `query` becomes short-hand `q`, snake case `array_filters`
   becomes `arrayFilters`
+
+  Example:
+
+      Mongo.update(MongoPool,
+        "test_collection",
+        query: %{foo => 4},
+        update: %{"$set": %{"modified_field": "new_value"}},
+        multi: true)
+
+        Mongo.update(MongoPool,
+          "test_collection",
+          query: %{foo: 4},
+          update: %{foo: 5, new_field: "new_value"}},
+          upsert: true)
+
+        Mongo.update(MongoPool, "test_collection", [
+          [q: %{foo: 24}, update: %{flag: "old"}],
+          [q: %{foo: 99}, update: %{luftballons: "yes"}, upsert: true]
+        ])
   """
+  @spec update(GenServer.server(), collection, [Keyword.t()], Keyword.t()) :: result(Mongo.UpdateResult.t())
+  def update(topology_pid, coll, updates, opts \\ [])
+
   def update(topology_pid, coll, updates, opts) do
     write_concern =
       filter_nils(%{
@@ -1170,11 +1192,12 @@ defmodule Mongo do
   end
 
   defp normalise_updates([[{_, _} | _] | _] = updates) do
-    updates
-    |> Enum.map(&normalise_update/1)
+    Enum.map(updates, &normalise_update/1)
   end
 
-  defp normalise_updates(updates), do: normalise_updates([updates])
+  defp normalise_updates([{_, _} | _] = updates), do: normalise_updates([updates])
+
+  defp normalise_updates(updates), do: updates
 
   defp normalise_update(update) do
     update

test/mongo_test.exs

@@ -527,6 +527,24 @@ defmodule Mongo.Test do
     end
   end
 
+  test "update", c do
+    coll = unique_name()
+
+    assert {:ok, _} = Mongo.insert_many(c.pid, coll, [%{foo: 42}, %{foo: 42}, %{_id: 1}])
+
+    assert {:ok, %Mongo.UpdateResult{acknowledged: true, matched_count: 2, modified_count: 2, upserted_ids: []}} = Mongo.update(c.pid, coll, q: %{foo: 42}, update: %{"$set": %{foo: 0}}, multi: true)
+
+    assert {:ok, %Mongo.UpdateResult{acknowledged: true, matched_count: 0, modified_count: 0, upserted_ids: []}} == Mongo.update(c.pid, coll, [query: %{foo: 0}, update: %{}], w: 0)
+
+    assert {:ok, %Mongo.UpdateResult{acknowledged: true, matched_count: 1, modified_count: 0, upserted_ids: [%BSON.ObjectId{}]}} = Mongo.update(c.pid, coll, query: %{foo: 100}, update: %{foo: 24, flag: "new"}, upsert: true)
+
+    assert {:ok, %Mongo.UpdateResult{acknowledged: true, matched_count: 2, modified_count: 1, upserted_ids: [%BSON.ObjectId{}]}} =
+             Mongo.update(c.pid, coll, [[q: %{foo: 24}, update: %{flag: "old"}], [q: %{foo: 99}, update: %{luftballons: "yes"}, upsert: true]])
+
+    # message: "Write batch sizes must be between 1 and 100000. Got 0 operations."
+    assert {:error, %Mongo.Error{code: 16}} = Mongo.update(c.pid, coll, [])
+  end
+
   # issue #19
   # test "correctly pass options to cursor", c do
   #  assert %Mongo.AggregationCursor{opts: [slave_ok: true, no_cursor_timeout: true], coll: "coll"} =