ruby string interpolation and dynamic programming

So I'm trying to be smart and make a generic method which can create the variable names and use them correctly. I am passing into 2 symbols into this method, and creating the correct variables and trying to use them.

 def are_numeric_fields_valid (column_name)
    single_name = column_name[0].to_s
    aggregate_name = column_name[1].to_s
    short_column_attribute = single_name.sub("_single", "")

    effective_on_attribute = :"#{short_column_attribute}_effective_on"
    expire_on_attribute = :"#{short_column_attribute}_expire_on"

      if ("loa_item.#{effective_on_attribute}".present? && "loa_item.#{expire_on_attribute}".present?) && (no_values_present?(single_name, aggregate_name))
            loa_item.errors.add(:base, "You must provide single and aggregate values when providing dates at the #{short_column_attribute} role level")
          end

end

But this always is true, even when the actual values are blank:

if ("loa_item.#{effective_on_attribute}".present? && "loa_item.#{expire_on_attribute}".present?)

Do I need something else so this will evaluate correctly? Like a send?