Custom Ecto.Type - how to create virtual fields to be used in select_merge/3?

Questions / Help
1

I have a use-case where I want a virtual ecto field to be used in select_merge/3:

schema "test" do
  field :test, Custom.Type, virtual: true
end

Then I want to do some custom data mapping on the received results on server-side(not database).

The problem is that none of the callbacks of the custom type are being fired. I would expect that the ones responsible for encoding when inserting data to not work, but not all of them.

Any ideas, suggestions on this behavior or ways to overcome it?

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2

Why would it be virtual if you load it from the db?

3

It’s not related in any way to the table structure, it’s a field that is a result of a joined relation + some postgres aggregate functions.

4

Schemas are already not (enforced to be) related to table structures in the first place.

5

Yes, however when doing inserts I want that field to be ignored, if I don’t set it as a virtual, ecto will complain that such field doesn’t exist.

6

Afaik that’s only supposed to happen if there are changes for that field.

7

It does seem to happen, the field is not even added to cast in changeset and Postgrex raises with column not found error, the query includes that column.

8

Is this on an update or an insert? You can insert with and without using a changeset. Though curious what value it even tries to insert.

9

On insert with changeset at the very least. It seems that changeset is irrelevant as Postgrex crashes when building the query, I cannot inspect the query even without running it.

10

Also on this topic, if I remember well virtual fields are not even validated when used together with select_merge, even though not using them will throw an error of invalid schema. :face_with_raised_eyebrow:

11

Small update: I have read ecto documentation and there is this option:

:load_in_query - When false, the field will not be loaded when selecting the whole struct in a query, such as from p in Post, select: p. Defaults to true.

It does seem to work on inserts and selects, the problem with select_merge and lack of any kind of validation still persists as before.

12

Recently, I wanted to load an aggregated JSONB array into a virtual field and to validate and cast it to a schema. The only way I got it working was via Ecto.Repo.load/2. It probably isn’t exactly what you’re looking for but might do the job.

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