Copying an existing Ecto struct for insertion

[Thomas J.]

A topic that comes back now and then, is how to copy an existing Ecto struct for insertion.

Example:
https://elixirforum.com/t/idiomatic-way-to-duplicate-an-ecto-changeset/33438  

https://stackoverflow.com/questions/38826848/how-to-duplicate-records-for-ecto 

 

Simply removing the `:id` field is not sufficient, because the Ecto struct contains meta data (e.g. `:loaded`). I run into errors relating to function clauses not matching when trying to insert an existing Ecto struct, even when removing the `:id` ; pretty sure because of the meta data.

This is how I solve the problem in my application:

```

def duplicate(foo) do
  data =
     foo
     |> Map.take(Foo.__schema__(:fields))
     |> Map.delete(:id)

  struct(Foo, data)
end

```

That seems to work fine. But note how I have to rely on reflection in order to achieve that.

Question/proposition: do you think we can offer a function for copying an existing Ecto struct (to be inserted)?

[José Valim]

We could add a new function to `Ecto`. The biggest question is how we would call it. :)

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[Thomas J.]

That is indeed a problem 😁 However, I'd say `copy_new/1` (as in "copy into new entity")

[Andrew Timberlake]

clone?

Andrew

[Thomas J.]

@Andrew for me the problem with copy/clone/duplicate/... is that it doesn't say that it becomes a new entity, that is, without the ID.

Hence I suggested to append `_new`

[José Valim]

The issue about specifying semantics, such as copy or copy_new is that you also need to reason what happens with associations (are they also freshened up?) and we may not have a universal answer.

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[Thomas J.]

Could then the way of handling association be specified through options and a default value is documented?

`copy_new/2` with option `:on_association` that may have the values `:copy_new`, `:keep`, `:unload`.

(quick sketchy idea dump)

[José Valim]

That could work, but it would have to be per association.

Further thoughts Regarding the name, I don’t like copy because every data change that we do in Elixir is a copy. It feels like we rather want to call it “erase_identity” or similar.

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[Adam Lancaster]

perhaps `new_from` or `build_from` ?

`build` is often what you see with factories for example, that build a struct but don’t put an id in it.

Best

Adam

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