unlogic
Question about Ecto dynamic and sortby
I’m trying to add a sort feature to an API which allows the user to pass the field name to sort by. I figured I could use dynamics as per https://hexdocs.pm/ecto/dynamic-queries.html#building-dynamic-queries but I run into an issue when adding the sort order specifier. this works fine:
dynamic([q], field(q, ^String.to_atom(sortby)))
this however gives an error
dynamic([q], desc: field(q, ^String.to_atom(sortby)))
** (Ecto.Query.CompileError) Tuples can only be used in comparisons with literal tuples of the same size
In order to get this working now I’ve used order_by instead of dynamic
order_by(query, [q], desc: field(q, ^String.to_atom(sortby)))
Which works, but I’d like to understand why the dynamic isn’t working for me, as it’s pretty much the same as in the documentation
Marked As Solved
LostKobrakai
So the problem is that the keyword syntax is special to the order_by macro. dynamic allows you to capture sql (query…) expressions. desc: field is not a query expression on it’s own, though. The keyword syntax is just a convenience to use elixir syntax with order_by. If you need to put the direction in the dynamic use plain sql possibly with an fragment.
def dynamic_order_expr(field, :asc),
do: dynamic([q], fragment("? ASC", field(q, ^field)))
def dynamic_order_expr(field, :desc),
do: dynamic([q], fragment("? DESC", field(q, ^field)))
Also Liked
LostKobrakai
Oh, that’s interesting. Maybe this is actually supposed to work (which I wouldn’t have expected). Please open an issue on ectos github, so either docs or the bug can be fixed.
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