Weird behaviour with Ecto.Schema.embeds_one/2

Questions / Help
1

I had a schema defined as below. Notice that I omitted the schema name for the embed.

defmodule MySchema do
schema "my_schema" do
 field :some_field, :string
 embeds_one :example_embed do
   field :test_field, :string
 end
end
end

This led to an error while casting the embed:

    ** (ArgumentError) errors were found at the given arguments:

  * 1st argument: not an atom

        :erlang.apply([do: :ok], :__schema__, [:primary_key])
        (ecto 3.8.4) lib/ecto/changeset/relation.ex:116: Ecto.Changeset.Relation.cast/5
        (ecto 3.8.4) lib/ecto/changeset.ex:810: Ecto.Changeset.cast_relation/4

Another weird thing was that when I inspect the fields in my schema, I find that the fields in the embeded schema are now part of the main schema:

%MySchema{
some_field: nil,
example_embed: nil,
test_field: nil
}

The correct way to do it is to have the embed schema name, like:

schema "my_schema" do
 field :some_field, :string
 embeds_one :example_embed, Example do
   field :test_field, :string
 end
end

Don’t you think this should give a warning? :thinking:

2

You forgot to add schema name

defmodule MySchema do
  schema "my_schema" do
    field :some_field, :string
    embeds_one :example_embed, Example do
      field :test_field, :string
    end
  end
end
3

Yes, exactly. But don’t you think it should have given a warning?

4

Well, it states exactly what is wrong. Second argument must be an atom (like Example), but it is a do block

5

Yeah, but I was wishing for something like you haven't specified a name for embedded schema during compilation

6

Also it defined the fields in the parent schema, which is awkward

7

It does not need to be so…

8

This is because your declaration ultimately invokes this branch of embeds_many:

with schema bound to the AST for a kwlist containing one option (do) and opts bound to [].

expand_alias doesn’t find the AST shape it’s looking for, so it returns schema unchanged.

The code in quote expands to (roughly):

Ecto.Schema.__embeds_one__(MySchema, :example_embed, [do: field(:test_field, :string)], [])

Before calling __embeds_one__, the arguments are evaluated, so field(:test_field, :string) runs in the context of the schema’s block - that’s why the field appears on the schema.

field always returns :ok, so __embeds_one__ is called with a “schema” that’s a keyword list [do: :ok] which cast_relation crashes on.

3 Likes
9

This makes a lot of sense now.