Ecto — select field or nil dynamically

Questions / Help
1

How to dynamically or conditionally select a value from a binding or nil?

def fetch_stuff(query, include_some_value?) do
  # SomeModel embeds SomeOtherModel which contains lots of stuff.
  # SomeModel.embedded is not required (default nil).
  preferably_nil_sometimes = (
    # binding.id (:integer) works but nil would be much better.
    if include_some_value?, do: :lots_of_stuff, else: :id
  )

  from [binding] in query,
  select: %SomeModel{
    id: binding.id,
    embedded: field(binding, ^preferably_nil_sometimes)
  }
end
2 
def fetch_stuff(query, false = _include_some_value?) do
  from(query, select_merge: %{embedded: nil})
end

def fetch_stuff(query, true = _include_some_value?) do
  # I'm not quite following if the query should be returned unchanged
  # like this or if there is other "stuff" to be done
  query
end
3

Thank you.

Actually SomeModel is also embedded and the source (binding) is not of the same type (schema).
Select_merge will complain about an incompatible merge.

those select expressions are incompatible. You can only select_merge:

  * a source (such as post) with another source (of the same type)
  * a source (such as post) with a map
  * a struct with a map
  * a map with a map

Also select_merge will delete any field not present in a map.
In this example SomeModel will not have an id because it will be deleted.

from [binding] in query,
select: %ParentModel{
  stuff: binding.parent_stuff,
  other_stuff: %SomeModel{
    id: binding.id,  # <-- `select_merge: %{embedded: nil}` will delete all but :embedded
    embedded: field(binding, ^preferably_nil_sometimes)
  }
}

The query should set :embedded to nil or to binding.lots_of_stuff.

4

Take a look at

5

Thank you, I have.

Using dynamic I’m not able to get the result I’m looking for.
I would like to select (embedded) structs not maps.
And I’d like to dynamically set only a single field, not an entire struct (or map).

6

Have you tried using struct/2, perhaps combined with select_merge?

1 Like
7

Struct/2 wants to infer the type (model/schema) from the source (binding).
I’m composing “custom” models using data from other sources.
So I guess that would require struct/3 which would be awesome :slight_smile: