Patch Rails :deep_munge issue so empty array parameters don't get converted to nil

Before people were unable to remove coordinator fees from an order cycle because Rails was converting the empty :coordinator_fee_ids array paramter into nil. This issue was introduced to Rails in v4.0.0.beta1 and isn't fixed until v5.0.0.beta1

Another way to fix this could be to do something like 'params[:coordinator_fee_ids] ||= []' but it seems like this issue could problems in other parts of the app so a more general fix might be better.

Fixes #6224

config/initializers/action_dispatch_request_deep_munge_patch.rb

@@ -0,0 +1 @@
+require "action_dispatch/request"

lib/action_dispatch/request.rb

@@ -0,0 +1,47 @@
+# This patch fixes the Rails issue where ActionDispatch::Request#deep_munge was converting empty
+# array paramters into nils, see https://github.com/rails/rails/issues/13420
+#
+# Before this patch:
+#
+# | JSON                             | Hash                    |
+# |----------------------------------|-------------------------|
+# | { "person": [] }                 | { 'person' => nil }     |
+#
+# After patch:
+#
+# | JSON                             | Hash                    |
+# |----------------------------------|-------------------------|
+# | { "person": [] }                 | { 'person' => [] }      |
+#
+# The issue started in Rails v4.0.0.beta1:
+#
+# https://github.com/rails/rails/commit/8e577fe560d5756fcc67840ba304d79ada6804e4
+#
+# This patch can be removed on or after Rails v5.0.0.beta1 when the issue was fixed:
+#
+# https://github.com/rails/rails/commit/8f8ccb9901cab457c6e1d52bdb25acf658fd5777
+#
+# Credit:
+#
+# https://gist.github.com/victorblasco/f675b4cbaf9c0bc19f81
+
+module ActionDispatch
+  class Request
+    def deep_munge(hash)
+      hash.each do |k, v|
+        case v
+        when Array
+          v.grep(Hash) { |x| deep_munge(x) }
+          v.compact!
+
+          # This patch removes the next line
+          # hash[k] = nil if v.empty?
+        when Hash
+          deep_munge(v)
+        end
+      end
+
+      hash
+    end
+  end
+end

spec/lib/action_dispatch/request_spec.rb

@@ -0,0 +1,18 @@
+require 'spec_helper'
+
+describe ActionDispatch::Request do
+  it "strips nils from arrays" do
+    assert_parses({ "key" => ["value"] }, 'key[]=value&key[]')
+    assert_parses({ "key1" => { "key2" => ["value"] } }, 'key1[key2][]=value&key1[key2][]')
+  end
+
+  it "doesn't convert an empty array to nil" do
+    assert_parses({ "key" => [] }, 'key[]')
+  end
+
+  private
+
+  def assert_parses(expected, actual)
+    assert_equal expected, ActionDispatch::Request.new('QUERY_STRING' => actual).query_parameters
+  end
+end