How to run a particular module's function only once when called in the context of different function variants?

Phoenix Forum Questions / Help
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1

I have this module used to filter Ecto results, the first function gets called 3 times, what modifications are needed to make it get called only once (details in code)?

defmodule MangoWeb.Filters.Borrowers do
  import Ecto.Query
  use Inquisitor

  # I want to run this only once when calling build_query(conn, _params) from the controller
  def build_query(query, _attrs, _values, _conn) do
    user = _conn.assigns[:current_user]
    from b in query,
      join: i in assoc(b, :institute),
      where: i.id == ^user.institute_id
  end

  # but the above gets called two additional times because of these two:
  def build_query(query, "code", code, _conn) when code != "" do
    Ecto.Query.where(query, [p], ilike(p.code, ^"%#{code}%"))
  end

  def build_query(query, "address", address, _conn) when address != "" do
    Ecto.Query.where(query, [p], ilike(p.address, ^"%#{address}%"))
  end
end

Thanks.

2

The thing that I note is that the more specific, or restricted, function calls (the one where you pattern match on “code” or “address”) are declared after your more generic function. It seems that the order of these should be reversed. You should put the more specific clauses of your function first and the more generic ones at the end.

Elixir will pattern match against your functions in the order that they are declared in the source.

2 Likes
3

I have tried to put it last, same result.

4

Can you then please share how you are calling the function?

PS: there are no methods in elixir, functions only.

1 Like
5

It looks like the library you are using (Inquisitor) calls your build_query/4 method once for each attribute passed in. Why do you want build_query/4 to only be called once?

6

I call it once in the controller action like thid:

Borrower
|> build_query(conn, _params)

It’s part of Inquisitor library.

7

For restricting the results based on some logged in user attribute, I believe it should be called once.

8

Depends on the content of _params.

So please tell us what the value of _params is.

Also, you shouldn’t use an underscore in front of the name when you actually use the variable.

1 Like
9 
%{
  "inserted_at" => "2017-10-01 00:00:00",
  "name" => "",
  "order_by" => "desc_inserted_at",
  "page" => "1",
  "per_page" => "20",
  "user_id" => ""
}

And here’s the function as defined in the library:

defmodule Inquisitor do

  defmacro __using__(_opts) do
    quote do
      def build_query(query, context, params) do
        Enum.reduce(params, query, fn({key, value}, query) ->
          build_query(query, key, value, context)
        end)
      end

      @before_compile Inquisitor
    end
  end

  defmacro __before_compile__(_env) do
    quote do
      def build_query(query, _key, _value, _context), do: query
      defoverridable [build_query: 4]
    end
  end
end
10

You got 6 entries in params, your build query should have been called 6 times.

The specific clauses do never match, so the generic one will be used.

1 Like