How to have custom enconding for struct using Jason?

Pff, my bad, I named my module in a wrong way, sorry for confusing. It had to be:

defmodule ElixirDraft.Json.JsonReader do
	alias ElixirDraft.Json.User
	
	def to_json(%User{} = user) do
		Jason.encode!(user)
	end	
	
	def to_model(json) do
		Jason.decode!(json)
	end
end

No warnings, it worked as expected :).

I have the same issue. If I get it right from the Consolidation docs, adding this to the mix.exs file would have fixed the warning:

def project do
  ...
  elixirc_paths: elixirc_paths(Mix.env())
  ...
end

defp elixirc_paths(:test), do: ["lib", "test/support"]
defp elixirc_paths(_), do: ["lib"]

But I still have it.
Here is how I defined my module and marked the fields to encode:

defmodule Draft.Json.User do
	
	@derive {Jason.Encoder, only: [:name, :email, :age]}
	defstruct [:name, :email, :age]
end

And here is a custom encoder module:

defmodule Draft.Json.JsonReader do
	alias Draft.Json.User
	
	def to_json(%User{} = user) do
		Jason.encode!(user)
	end	
	
	def to_model(json) do
		Jason.decode!(json)
	end
end

When starting an IEx session, the warning pooped up as before:

Erlang/OTP 27 [erts-15.0] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [jit]

    warning: the Jason.Encoder protocol has already been consolidated, an implementation for Draft.Json.User has no effect. If you want to implement protocols after compilation or during tests, check the "Consolidation" section in the Protocol module documentation
    │
  4 │ 	defstruct [:name, :email, :age]
    │         ~~~~~~~~~~~~~~~~~~~~~~~~
    │
    └─ lib/json/user.ex:4: Draft.Json.User (module)

What am I missing?

Thank you!

Docs state advantage of protocol consolidation as below:

Consolidation directly links protocols to their implementations in a way that invoking a function from a consolidated protocol is equivalent to invoking two remote functions.

For solving the problem in original post - you have to create the module and encoder in a file say test.ex in lib folder and it will work.

@kartheek great answer, and I very much enjoyed the link to the book.

One last thing though, just to confirm, the only reason for not doing option 1, is because test performance can be affected, correct?

• Protocol — Elixir v1.20.2

As for the proposed solution, how would I go about it?

def project do
  ...
  elixirc_paths: elixirc_paths(Mix.env())
  ...
end

defp elixirc_paths(:test), do: ["lib", "test/support"]
defp elixirc_paths(_), do: ["lib"]

I understand I need to have a "test/support/something.ex" file, but it is not clear to me what the contents of this file would be.

You can read about Protocol Consolidation.

Protocol consolidation happens at compile time. You can disable or enable from mix config as mentioned in docs.

When it is enabled (by default) - it will link protocols with their implementations at compile time to optimise invocations. So when you are trying it in iex - it has no effect as protocol consolidation already happened.

Two solutions:

• disable protocol consolidation using mix config (not recommended) - Section 16.6 don’t go by title read the content in linked section in this book - https://www.elixircryptobot.com/.
• define all these encoders in files and compile the project (recommended)

No problem with the code or library or project - its working as it is supposed to be.

@kartheek and @thomasbrus
I am trying this directly in iex, yes. I read the comment, but I don’t quite understand what it means.
I also don’t quite understand how this affects the code I am writing. If someone could explain, I would be thankful.

Did you see this warning by any chance?

warning: the Jason.Encoder protocol has already been consolidated, an implementation for Test has no effect. If you want to implement protocols after compilation or during tests, check the “Consolidation” section in the Protocol module documentation
dummy.exs:12: Test (module)

(meant to reply to @Fl4m3Ph03n1x)

I stand corrected, thanks @LostKobrakai !

That’s not required if the implementation is nested within the module the implementation is for.