Given example:
I have two models with standart many_to_many association and join table.

defmodule Tag do
  ...
  schema "tags" do
    field :name, :string

    many_to_many :posts, Post, join_through: "posts_tags"
  end
end

defmodule Post do
  ...
  schema "posts" do
    ...
    many_to_many :tags, Tag, join_through: "posts_tags",
  end
end

Join table :posts_tags, has
  :post_id, references(:posts)
  :tag_id, references(:tags)

And I have select form with multiselect for filter posts by tags:
= multiple_select f, :tags, Enum.map(@tags, &{&1.name, &1.id})

Now I do smth like this for filtering:

def list_posts(params)
  tags_params = Map.get(params, "tags")
  filtrex_params = Map.drop(params, ["tags"])

  case Filtrex.parse_params(config, filtrex_params) do
    {:ok, filter} -> Filtrex.query(Post, filter)
    {:error, error} -> {:error, error}
  end
  |> custom_filters(tags_params) |> Repo.all
end

def custom_filters(query, nil), do: query
def custom_filters(query, tags) do
   tags_ids = Enum.map(tags, &String.to_integer(&1))
   from p in query, join: t in assoc(p, :tags), group_by: p.id, having: fragment("? <@ array_agg(?)", ^tags_ids, t.id)
end

It works fine for many_to_many and multi select.
Have better solution?
@rcdilorenzo

Support for associations #38

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