Always keep map literal in select_merge if keys are distinct

[greg-rychlewski]

Previously we discussed if insert_all can support select_merge with multiple map literals when one of the inner maps has interpolations. For example:

from p in "posts", select: %{title: ^"title"}, select_merge: %{visits: 1}

Because the inner map has an interpolation, we don't know whether it will be wiped out and our query params will be incorrect. So we turn the select expression into merge(%{...}, %{....}). And this merge expression doesn't work with insert_all due to difficulty extracting fields from merge.

There is one condition we can be certain the interpolation won't be wiped out: when all the merges have distinct keys. In this case we can keep the map literal and it will work with insert_all.

This would give a lot more flexibility to user. If they have dynamic keys/values they want to turn into multiple merge statements, they can always remove duplication in Elixir before going to Ecto land.

For example this:

conditions = [k1: v1, k2: v2, k3: v3, ...]
base = from p in "posts", select: %{k0: v0}

Enum.reduce(conditions, base, fn {k, v}, query -> from q in query, select_merge: %{^k => ^v}

They can ensure no duplications in conditions beforehand.

[josevalim]

Really nice improvement! ❤️