I messed up. fields is already translated to :source. I'll try to see if there's anything else I can do.

@josevalim thanks for the great feedback. i believe i addressed all of it :)

@greg-rychlewski I have been thinking about this PR and I am not really sure if we should support the update the syntax. The biggest issue is that, if you write this:

select: %{p | foo: 321}

It tells me you are selecting *all of p and updating the value of :foo. The fact we are selecting only a subset is a bit awkward to me, even though it is the most logical. What would we do if the user really wants to select all of them in the future? Sorry for not noticing this earlier, but we can probably keep the other warts of this PR (except the update)?

@josevalim I'll get the read_only -> writeable change done this week so we are not stuck not being able to release for long. In case someone asks you to