Having wrong before and after key values whenever I'm using select in query

[undefhero]

We've faced in a problem while we are using Paginator with Ecto.Query.select/3
There is a code sample:

query = from u in User, order_by: [asc: user.inserted_at, asc: user.id], select: %{user: u}
Backend.Repo.paginate(query, cursor_fields: [inserted_at: :asc, id: :asc])

Got a metadata:

%Paginator.Page.Metadata{
    after: "g3QAAAACZAACaWRkAANuaWxkAAtpbnNlcnRlZF9hdGQAA25pbA==",
    before: nil,
    limit: 2,
    total_count: 20,
    total_count_cap_exceeded: false
}

And after I applied it to fetch a next page I got it but before and after keys have the same values as a previous Metadata has.

%Paginator.Page.Metadata{
    after: "g3QAAAACZAACaWRkAANuaWxkAAtpbnNlcnRlZF9hdGQAA25pbA==",
    before: "g3QAAAACZAACaWRkAANuaWxkAAtpbnNlcnRlZF9hdGQAA25pbA==",
    limit: 2,
    total_count: 20,
    total_count_cap_exceeded: false
}

Any workaround here?

[undefhero]

I also tried with :fetch_cursor_value_fun and had a similar result.

%Paginator.Page.Metadata{
    after: "g3QAAAABZAALaW5zZXJ0ZWRfYXR0AAAACWQACl9fc3RydWN0X19kABRFbGl4aXIuTmFpdmVEYXRlVGltZWQACGNhbGVuZGFyZAATRWxpeGlyLkNhbGVuZGFyLklTT2QAA2RheWEeZAAEaG91cmEOZAALbWljcm9zZWNvbmRoAmEAYQBkAAZtaW51dGVhBGQABW1vbnRoYQtkAAZzZWNvbmRhMGQABHllYXJiAAAH5Q==",
    before: "g3QAAAABZAALaW5zZXJ0ZWRfYXR0AAAACWQACl9fc3RydWN0X19kABRFbGl4aXIuTmFpdmVEYXRlVGltZWQACGNhbGVuZGFyZAATRWxpeGlyLkNhbGVuZGFyLklTT2QAA2RheWEeZAAEaG91cmEOZAALbWljcm9zZWNvbmRoAmEAYQBkAAZtaW51dGVhBGQABW1vbnRoYQtkAAZzZWNvbmRhMGQABHllYXJiAAAH5Q==",
    limit: 2,
    total_count: 20,
    total_count_cap_exceeded: false
  }

[sgerrand]

Thanks for raising this issue! Are you able to provide a small dataset to help us verify this bug?

[undefhero]

@sgerrand thank you for your response. I was using the paginator database to confirm the issue

query = from(c in Payment, order_by: [asc: c.inserted_at, asc: c.id], select: %{c: c})

%{metadata: %{after: cursor}} = Repo.paginate(query, cursor_fields: [inserted_at: :asc, id: :asc], limit: 3)
%{metadata: %{before: ^cursor, after: ^cursor}} = 
  Repo.paginate(query, cursor_fields: [inserted_at: :asc, id: :asc], after: cursor, limit: 3)

https://github.com/undefhero/paginator/blob/8f1841f6330c5a9aaa59d6d9a240583a910d4dd7/test/paginator_test.exs#L667

[nicolasblanco]

Hello!

I can confirm this bugs happens in my program when I'm using a select in my query.

If I can help debug this issue, tell me.

[nicolasblanco]

I have found the solution.

You just need to be sure that in the select of the query you correctly have at least all the fields that you use in the cursor_fields option.

It worked for me 🙌🏻 .