Ecto: Allow schema :field to have an alias

[Bharat311]

Hi,

I am fairly new to elixir and am currently working on developing a service application on phoenix.

It uses MongoDB as a backend and i am using mongo-ecto as the adapter to connect to the database.

The issue i am facing is that the fields in the MongoDB (as per normal practise) use short names. So, first_name is stored as fn, etc. Now, I want to access the field using proper names (read: first_name) throughout the code.

One approach that i could think of makes use of virtual fields like below:

defmodule User do
  use Blog.Web, :model

  schema “users” do
    field :fn, :string
    field :first_name, :string, virtual: true
  end

  def copy_from_alias(%{})
    # we copy values from alias
  end

  def copy_to_alias(%{})
    # we set value to aliases 
  end
end

We can also have the above code autogenerated via a macro like aliased_field.

However, with this i’ll need to explicitly call these methods whenever casting data to/from database.
Is there any way to avoid doing that ?

Also, i am not sure how to get it working with queries. For eg:

Repo.get_by(User, %{first_name: “Bharat”})

should check for fn when forming query and not first_name.

Is there any other approach that solves this problem ? I am willing to raise a PR for the same as i believe this to be a fairly regular use case (MongoDB, legacy databases, etc.) if i can get some pointers.

Any help is appreciated.

[johnnyshields]

Two proposals:

  schema "users" do
    field :fn, :string, alias: :first_name
  end

or

  schema "users" do
    field :first_name, :string
  end

  mongo_adapter "users" do
     map_field :first_name, to: :fn
  end

[johnnyshields]

@josevalim would be great to get your direction on how this could be implemented, we're happy to give it a try if there's an agreed upon spec.

[johnnyshields]

@josevalim any comment here? This is an important issue for our team to migrate from Ruby/Mongoid to Elixir.

[sevenseacat]

Out of curiosity, why is it 'normal practise' to use cryptic and meaningless field names in MongoDB?

[johnnyshields]

@sevenseacat unlike other DBs which use position-dependent columns, MongoDB is a document database and the field (column) names are stored within the document (row). So you can significantly reduce the size of objects by using aliases:

{ my_long_field_name: "my value", my_other_field: "other value" }

becomes

{ f1: "my value", f2: "other value" }

[johnnyshields]

A secondary reason for both schema and schema-less DBs is from time-to-time you may want to migrate or rename columns, and using aliases make it much easier to move things around without renaming the variable throughout your entire app.

[sevenseacat]

Cheers for that. Now I have another reason to avoid Mongo apps...

[josevalim]

@johnnyshields I believe this has been discussed in the past. Did you find any relevant information in previous discussions?

[johnnyshields]

@josevalim I'm not aware of where these discussions were held. I've searched this repo for issues containing "alias" and I don't see it.

[josevalim]

@johnnyshields let's give the :alias option a try then. This is a somewhat complex issue because it does not only touch insert/update/delete operations but also all queries.

[johnnyshields]

@josevalim great, thanks. We'll start work on this soon.

[Qqwy]

Just wanted to make sure that you are aware of the discussion about aliases that is going on on the Elixir forum. I think this is mostly relevant because the Original Poster describes an alternative situation in which field aliases would be helpful, namely one in which Elixir+Ecto needs to use an already-existing database that uses CamelCase field names.

[hbobenicio]

+1 on this.
In my case, my database has "ugly" column name conventions that I wouldn't like to inherit in my phoenix/ecto project. I got some columns like "some_random_prefix_convention_foo" that would be great if used only as "foo" on my model.

[johnnyshields]

@hbobenicio there is a PR branch here https://github.com/elixir-ecto/ecto/pull/1998 ready to merge which we are using in our apps, please help push to get it merged.

[josevalim]

In master! Please give it a try and test it further!

[scf4]

Is there a way to convert names between camel/snake case, without "source" on every field?

[michalmuskala]

You could create a custom field macro that would compute the correct source and call the original field.

[josevalim]

The macro works but we can possibly make it easier. What if we have a
source_mapper option that is given to schema/2 and it I invoked for every

field?

José Valimwww.plataformatec.com.br
http://www.plataformatec.com.br/Founder and Director of R&D

[scf4]

Thanks @michalmuskala I'll give it a try

@josevalim a source_mapper option would be perfect!