How to select_merge a dynamic binding into a map with variable keys?

Questions / Help
1

Hello!

Given a query with N joins at runtime

query = from(e in Entity)

assocs = [:assoc_a, :assoc_b, :assoc_c, ...] # list of unknown length

query = assocs
|> Enum.reduce(query, fn assoc_name, acc_query -> 
  join(acc_query, :left, [e], assoc(e, ^assoc_name), as: ^assoc_name)
end)

Is it possible to map the entire joined relation back onto the return value using select_merge like so?

assocs
|> Enum.reduce(query, fn assoc_name, acc_query -> 
  select_merge(acc_query, [{^assoc_name, bind}], %{^assoc_name => bind})
end)

If not, is there any way to construct a select statement that returns all of the associations at runtime, without prior knowledge of the total number of joined relations?

The project I’m working on involves a dynamic query constructor core built using macros and my feeling is that we are essentially re-inventing preloads… But I’d like to avoid refactoring our codebase if at all possible since the number of tables we have to touch is quite high

2

Hello and welcome!

Why not just use preloads? You should just be able to pass your initial assoc list to Repo.preload.

3

Yes preload would definitely be the sensical option haha

The project was just written in a way where the select statement is generated, and there is a bunch of complexity around how the results are then patched back onto the data in a different layer

Basically we should probably use preload, but that would have a significant time cost for refactoring, and I’m curios if there’s anyway to avoid that haha

Is it not possible to use select_merge and/or dynamic statements in that way?

4

Have you tried map? I do similar but I flatten them all into the one map. It looks like you can handle preloads from the docs.

https://hexdocs.pm/ecto/Ecto.Query.API.html#map/2

1 Like
5

Hey thanks for the suggestions @sodapopcan and @cmo

Ultimately I bit the bullet and and refactored the code base to leverage Ecto properly.

@cmo I had looked at map previously but it’s not quite the right fit, unless I’m mistaken we can only use it to pull fields out of a binding, as opposed to set a binding to a new field.

Thank you both for your suggestions!