How can I get the columns used in the where expression from an existing ActiveRecord_Relation?

Say a lot of my models implement a class method self.foo which returns a ::ActiveRecord_Relation.

From elsewhere in the code, I have access to the Model or the ::ActiveRecord_Relation returned by that method, and I would like to find out which columns are used in where expressions in the query.

If the query is simple, I can get it from where_values_hash on the ActiveRecord_Relation, but I'm having trouble figuring out how to get all the columns in more complex queries.

class A < ApplicationRecord
  def self.foo(bar)
    where(column1: bar)
  end
end

class B < ApplicationRecord
  def self.foo(bar)
    joins(:c).
    merge(C.where(column2: bar))
  end
end

elsewhere

# this evaluates to ["column1"] - success!
A.foo(bar).where_values_hash.keys 

# How can I get "c" or C and "column2" from B or B.foo(bar)?
# B or B.foo(bar).??? -> ["c", "column2"]

I could always get it from parsing the relation's .to_sql string, but there's got to be a better way.