Ecto use of ... (last binding) in dynamic as part of ON clause

Demonstration of bug in Ecto's dynamic when used with "last binding" operator as ON clause of the constructed JOIN statement

Use case

We want to dynamically add multiple INNER JOIN clauses to the query based on many filters from the user request. Each JOIN has different ON clause that is referencing that particular JOINED table.

Problem

Using join_cond = dynamic([c, ..., last_join]) in on: join_cond results in that the last_join binding is calculated to the previous binding, instead of the current join table where on: is being added.

Example:

query = from c in Customer,
  where: c.name == "Test Customer"

# construct "ON" condition as conjunction of all params on the last JOIN binding
# the `ev` here should be calculated as `e1`
join_on = dynamic([c, ..., ev], ev.name == "Event 1" and ev.location == "Location 1")

# append the inner join to the query, with extra ON condition from dynamic above
query = from c in query,
  inner_join: ev in assoc(c, :events),
  on: ^join_on # the `ev` binding from dynamic will be here expanded as `c`

# add one more join. this time `ev` here should be calculated as `e2`
join_on = dynamic([c, ..., ev], ev.name == "Event 2" and ev.location == "Location 2")

query = from c in query,
  inner_join: ev in assoc(c, :events),
  on: ^join_on # and here the `ev` binding will be here expanded as `e0` - ie `ev` from the previous inner_join

# Expect the query be successfull, with each JOIN's ON condition
# reference its JOIN binding
result = App.Repo.all(query)

Run

mix deps.get
mix ecto.create
mix ecto.migrate
mix test

What happens:

     ** (Ecto.QueryError) deps/ecto/lib/ecto/association.ex:509: field `location` in `join` does not exist in schema App.Customer in query:

     from c in App.Customer,
       join: e0 in App.Event,
       on: e0.customer_id == c.id and (c.name == ^"Event 1" and c.location == ^"Location 1"),
       join: e1 in App.Event,
       on: e1.customer_id == c.id and (e0.name == ^"Event 2" and e0.location == ^"Location 2"),
       where: c.name == "Test Customer",
       select: c

Notice that the first join: e0 has on: clause with c.name, and the second join: e1 has on: clause with the e0.name.

What should happen:

The query should look like this:

     from c in App.Customer,
       join: e0 in App.Event,
       on: e0.customer_id == c.id and (e0.name == ^"Event 1" and e0.location == ^"Location 1"),
       join: e1 in App.Event,
       on: e1.customer_id == c.id and (e1.name == ^"Event 2" and e1.location == ^"Location 2"),
       where: c.name == "Test Customer",
       select: c