dynamic_filters.ex

def filter(params) do
  Post
  |> order_by(^filter_order_by(params["order_by"]))
  |> where(^filter_where(params))
  |> where(^filter_published_at(params["published_at"]))
end
 
def filter_order_by("published_at_desc"), do: [desc: :published_at]
def filter_order_by("published_at"),      do: [asc: :published_at]
def filter_order_by(_),                   do: []
 
def filter_where(params) do
 for key <- [:author, :category],
   value = params[Atom.to_string(key)],
   do: {key, value}
end
 
def filter_published_at(date) when is_binary(date),
  do: dynamic([p], p.published_at > ^date)
def filter_published_at(_date),
  do: true