update

phoenix/deps/ecto_sql/integration_test/sql/lock.exs

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+defmodule Ecto.Integration.LockTest do
+  # We can keep this test async as long as it
+  # is the only one accessing the lock_test table.
+  use ExUnit.Case, async: true
+
+  import Ecto.Query
+  alias Ecto.Integration.PoolRepo
+
+  defmodule LockCounter do
+    use Ecto.Schema
+
+    schema "lock_counters" do
+      field :count, :integer
+    end
+  end
+
+  setup do
+    PoolRepo.delete_all(LockCounter)
+    :ok
+  end
+
+  test "lock for update" do
+    %{id: id} = PoolRepo.insert!(%LockCounter{count: 1})
+    pid = self()
+
+    lock_for_update =
+      Application.get_env(:ecto_sql, :lock_for_update) ||
+      raise ":lock_for_update not set in :ecto application"
+
+    # Here we are manually inserting the lock in the query
+    # to test multiple adapters. Never do this in actual
+    # application code: it is not safe and not public.
+    query = from(lc in LockCounter, where: lc.id == ^id)
+    query = %{query | lock: lock_for_update}
+
+    {:ok, new_pid} =
+      Task.start_link fn ->
+        assert_receive :select_for_update, 5000
+
+        PoolRepo.transaction(fn ->
+          [post] = PoolRepo.all(query) # this should block until the other trans. commit
+          post |> Ecto.Changeset.change(count: post.count + 1) |> PoolRepo.update!
+        end)
+
+        send pid, :updated
+      end
+
+    PoolRepo.transaction(fn ->
+      [post] = PoolRepo.all(query)       # select and lock the row
+      send new_pid, :select_for_update   # signal second process to begin a transaction
+      post |> Ecto.Changeset.change(count: post.count + 1) |> PoolRepo.update!
+    end)
+
+    assert_receive :updated, 5000
+
+    # Final count will be 3 if SELECT ... FOR UPDATE worked and 2 otherwise
+    assert [%LockCounter{count: 3}] = PoolRepo.all(LockCounter)
+  end
+end