Make it easier to work with Time, Date and DateTime Ecto types

[paulcsmith]

See #54 for why this would be nice.

I think it would be good to add to_date_time, to_date, to_time and corresponding sigil_t sigil_d and sigil_(not_sure_yet) sigils that can be imported and would convert a regular Erlang timestamp to the appropriate Ecto type. I imagine you could do something like this to make it easy to use:

defmodule MyApp.Factory do
  # This would import the functions for use inside the factory
  use ExMachina.Ecto, repo: MyRepo

  # I think this is nice because importing the sigils is optional so there are no accidental collisions
  defmacro __using__(_opts) do
    quote do
      import MyApp.Factory
      # Imports the sigils so you can them in your tests
      import ExMachina.TimeHelpers
    end
  end
end

# In a test

use MyApp.Factory # import the factory and the time helpers
create(:article, inserted_at: ~t(GoodTimes.a_day_ago)) # GoodTimes generates erlang time stamps

[paulcsmith]

I believe I misunderstood the capabilities of sigils. For example

date_time = GoodTimes.a_day_ago
~d(date_time)

Gets passed to sigil_d as {:<<>>, [line: 21], ["date_time"]} without any other information that would make it possible to figure out what date_time refers to. It could work with tuples and strings, but not variables like in that example.

So I think the sigil route is too limited to be that useful. We could make it work with strings, but I don't think that's super useful. I think it might be useful to have some shortcut functions like dt, t, and d cast to Ecto.DateTime, Ecto.Time and Ecto.Date respectively

[paulcsmith]

I may delay this and wait for better date time support to become standard in Ecto now that Elixir will have native calendar types: elixir-lang/elixir#4383