Polymorphic embed in Ecto

alexcastano

Hello,

I want to store a structured raw input in a JSONB column. This raw input is already in an embedded schema because it has been previously validated. However, the raw input can be of different type, some of the types being quite complex, as they have deeply nested structures.

When I had only 4 different types, I had a column for each one and I handled the polymorphism by myself:

schema "table" do
  ...
  embeds_one type_one, TypeOne
  embeds_one type_two, TypeTwo
  ...
end

But now, I want to add a lot more types, and I don’t think is efficient to have 30 different columns. So I would like to store the information in the same column, but when loading the record using a type field, it would load one structure or another.

I’m trying to use Ecto.Type to do this job, but I find myself doing a lot of work that was previously handled by Ecto:

First, I have to @derive {Jason.Encoder, only: [...]} in a lot of schemas. It feels wrong because I’m forced to use this protocol to store this information in one way. If I want to use the same protocol for another use, for example to encode in a JSON API, I won’t be able to do it in the future.

Also, I’m creating a lot of functions to load the raw JSON in the structures, basically using cast and cast_embed with all the stored fields.

So, I don’t know if I’m missing something, but I’d like to use functionalities that are already in Ecto with embeds_one to load, dump, embed_as, etc. my custom Ecto.Type. Do you think it would be possible?

Do you think is a good solution in general terms? Any advice?

9 comments

#ecto #ecto_sql

1 4310 9

2020-05-30 16:15:04 UTC

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mathieuprog

I published a library that brings support for polymorphic embeds

https://github.com/mathieuprog/polymorphic_embed

Post #9

Eiji

@alexcastano Of course it’s possible, but ecto itself does not helps in complex polymorphic use cases, so you would need to write it yourself.

First of all you would need to have your JSON like:

{"data": …, "type": "your_type_name"}

For it you would need to create a custom Ecto.Type. For each callback you need to check type field and based on it you would need to create n number of modules which also implements Ecto.Type.

Let’s say:

defmodule MyApp.EctoTypes.MainJSON do
  use Ecto.Type

  def ecto_callback(%{data: data, type: type}) do
    with {:ok, result} <- type |> find_module() |> :apply(:ecto_callback, [data]) do
      %{data: result, type: type}
    end
  end

  defp find_module("first_type"), do: MyApp.EctoTypes.FirstType
  defp find_module("second_type"), do: MyApp.EctoTypes.SecondType
  defp find_module("third_type"), do: MyApp.EctoTypes.ThirdType
end

defmodule MyApp.EctoTypes.FirstType do
  # no need to use Ecto.Type here
  # as this those "sub types" would be used only for Kernel.apply/3
  # in order to simply separate code for each type

  def ecto_callback(data) do
    # …
  end
end

In such way you can simply write a code for each type. You only need to implement each Ecto.Type required callback (cast, dump and load if I remember correctly).

If you do this all you need to do (like migrations) is exactly the same as with embeds_many or embeds_one. I wrote all from memory, but it should work even with arrays.

defmodule MyApp.MyContext.MySchema do
  alias MyApp.EctoTypes.MainJSON

  schema "table_name" do
    …
    field(:field_name, {:array, MainJSON})
    …
  end
end

The only difference is that you no longer need to call cast_embed and related functions as everything would be handled in cast.

Post #1

al2o3cr

Have you looked at GitHub - greenboxal/ecto_poly: Polymorphic embeds for Ecto · GitHub ? I haven’t tried it yet, but it seems pretty close to what you’re looking for.

EDIT: looks like Ecto 3 compatibility is still in-flight Ecto 3.x by jeanparpaillon · Pull Request #1 · greenboxal/ecto_poly · GitHub

Post #3