How do I use a dynamic comparison operator in Elixir Ecto?

So I'm trying to allow search on various fields in a database, this is the situation:

1. A user wants to find all posts that were created by users who joined before October 2020
2. A user wants to find all posts that were created by users who joined after October 2020
3. A user wants to find all posts that were created by users who joined on October 5th 2020

normally I would need to write various verions of this query like so:

def before_date(query, date) do
 query
 |> where([posts, users], users.inserted_at < ^date))
end 
def after_date(query, date) do
 query
 |> where([posts, users], users.inserted_at > ^date))
end 
def on_date(query, date) do
 query
 |> where([posts, users], users.inserted_at == ^date))
end

Then to filter on the posts date themselves, I would need to write those queries over again and uses posts.inserted_at instead.

Therefore I'm looking for a way to dynamically compare items inside and Ecto Query.

I found this question: Create Ecto query with dynamic operators

but it didn't compile as it complained that the query was not valid:

== Compilation error in file lib/mmsapi/liquid/search/filters.ex ==
** (CompileError) lib/mmsapi/liquid/search/filters.ex:338: invalid call operator(field(o, ^field_name), ^value)
    expanding macro: Liquid.Search.Filters.custom_where/4

so a slight modification and got to this, by replacing the variable operator with an actual operator:

defmacrop custom_where(t, f, v, :==) do
 {:==, [context: Elixir, import: Kernel],
  [
    {:field, [], [t, {:^, [], [f]}]},
    {:^, [], [v]}
   ]}
end

def compare_field(query, field_name, value, operator) do
  query
  |> where([o], ^custom_where(o, field_name, value, operator))
end

However with that I get this error:

== Compilation error in file lib/mmsapi/liquid/search/filters.ex ==
** (CompileError) lib/mmsapi/liquid/search/filters.ex:327: cannot use ^field_name outside of match clauses

This is the kind of code I'm trying to achieve:

 field_name = :inserted_at

 value = DateTime.utc_now()

 operator = :==

 query
 |> where([posts, users], custom_where(users, field_name, value, operator))

Any assistance would be most appreciated, and thank you in advanced!