Allow redefining to_param delimiter using param_delimiter

This commit allows customizing the delimiter used by `to_param` when
`to_key` returns multiple value. This unblocks supporting more varieties
of composite primary key types in Active Record.

Author: nvasilevski

activemodel/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Add `ActiveModel::Conversion.param_delimiter` to configure delimiter being used in `to_param`
+
+    *Nikita Vasilevsky*
+
 *   `undefine_attribute_methods` undefines alias attribute methods along with attribute methods.
 
     *Nikita Vasilevsky*

activemodel/lib/active_model/conversion.rb

@@ -24,6 +24,14 @@ module ActiveModel
   module Conversion
     extend ActiveSupport::Concern
 
+    included do
+      ##
+      # :singleton-method:
+      #
+      # Accepts a string that will be used as a delimiter of object's key values in the `to_param` method.
+      class_attribute :param_delimiter, instance_reader: false, default: "-"
+    end
+
     # If your object is already designed to implement all of the \Active \Model
     # you can use the default <tt>:to_model</tt> implementation, which simply
     # returns +self+.
@@ -80,7 +88,7 @@ def to_key
     #   person = Person.new(1)
     #   person.to_param # => "1"
     def to_param
-      (persisted? && key = to_key) ? key.join("-") : nil
+      (persisted? && key = to_key) ? key.join(self.class.param_delimiter) : nil
     end
 
     # Returns a +string+ identifying the path associated with the object.

activemodel/test/cases/conversion_test.rb

@@ -53,4 +53,25 @@ def persisted?
   test "to_partial_path handles namespaced models" do
     assert_equal "helicopter/comanches/comanche", Helicopter::Comanche.new.to_partial_path
   end
+
+  test "#to_param_delimiter allows redefining the delimiter used in #to_param" do
+    old_delimiter = Contact.param_delimiter
+    Contact.param_delimiter = "_"
+    assert_equal("abc_xyz", Contact.new(id: ["abc", "xyz"]).to_param)
+  ensure
+    Contact.param_delimiter = old_delimiter
+  end
+
+  test "#to_param_delimiter is defined per class" do
+    old_contact_delimiter = Contact.param_delimiter
+    custom_contract = Class.new(Contact)
+
+    Contact.param_delimiter = "_"
+    custom_contract.param_delimiter = ";"
+
+    assert_equal("abc_xyz", Contact.new(id: ["abc", "xyz"]).to_param)
+    assert_equal("abc;xyz", custom_contract.new(id: ["abc", "xyz"]).to_param)
+  ensure
+    Contact.param_delimiter = old_contact_delimiter
+  end
 end