order_by with dynamic value on Ecto

Asked Aug 02 '18 at 12:51

Active Aug 02 '18 at 17:09

Viewed 647 times

I'm trying to order by a string provided dynamically. The field I want to order is alias in the select.

query
|> select([t], %{orders_count: fragment("? AS orders_count", count(t.id)})
|> order_by(fragment("?", ^field))

This will generate this, and will not work.

order_by: [asc: fragment("?", ^"orders_count")],

But if I put the string name without a variable

query
|> select([t], %{orders_count: fragment("? AS orders_count", count(t.id)})
|> order_by(fragment("orders_count"))

will generate this, and it works.

 order_by: [asc: fragment("orders_count")],

But I can't understand why it doesn't work with the first example.

edited Aug 02 '18 at 17:09

asked Aug 02 '18 at 12:51

David Magalhães

@mudasobwa, it doesn't compile. It says undefined function `field/0`. – David Magalhães Aug 02 '18 at 13:06
Wild guess was wrong then :( – Aleksei Matiushkin Aug 02 '18 at 13:16
What's the value of `field` here? The string `"orders_count"`? – Dogbert Aug 02 '18 at 13:30
@Dogbert, yes. And other possible string values. I can add all fields manually (it's 3), but in terms of code isn't pretty. – David Magalhães Aug 02 '18 at 13:32
I was trying to get some way to call `fragment("orders_count")` dynamically, https://stackoverflow.com/questions/13230965/how-to-call-a-method-dynamically-in-elixir-by-specifying-both-module-and-method but I thinks it's way over my head. Ecto is also pretty complex. – David Magalhães Aug 02 '18 at 13:48
I don't think this is possible because `fragment` doesn't accept a runtime string as the first value to prevent SQL injection. See the error message thrown when you do `order_by(fragment(^field))`. – Dogbert Aug 02 '18 at 13:55
If the field is a member of the struct, you can do `order_by([p], field(p, ^:"#{field}"))`. – Dogbert Aug 02 '18 at 13:56
It isn't a member of struct, it's on the select with an alias. – David Magalhães Aug 02 '18 at 13:57

0 Answers0