Fragment constant/1 doesn't need adapter

[greg-rychlewski]

Right now we are passing {:constant, [], [value]} to the adapter and the adapter is taking care of the unpacking. But we can pass the constant value directly from Ecto and the existing adapter functionality can take care of it without the need for handling a :constant tuple.

[greg-rychlewski]

The integration test failure is related to TDS and how the adapter was handling :constant in the past. It looks like it wasn't injecting strings correctly.

The failure:

* |   1) test fragments (Ecto.Adapters.TdsTest)
     +integration-test *failed* |      test/ecto/adapters/tds_test.exs:690
     +integration-test *failed* |      Assertion with == failed
     +integration-test *failed* |      code:  assert all(query) == ~s"SELECT 'let''s escape' FROM [schema] AS s0"
     +integration-test *failed* |      left:  "SELECT N'let''s escape' FROM [schema] AS s0"
     +integration-test *failed* |      right: "SELECT 'let''s escape' FROM [schema] AS s0"

The reason is because string literals are injected this way:

defp expr(string, _sources, _query) when is_binary(string) do
    "N'#{escape_string(string)}'"
end

But :constant was injected this way

defp expr({:constant, _, [literal]}, _sources, _query) when is_binary(literal) do
  [?', escape_string(literal), ?']
end

I can fix the test in the an ecto sql pr

[josevalim]

Sounds good!