Serialize nested Her::Model instances that are not associated

We previously added serialization for ActiveModel::Serialization
instances but Her::Model instances can still be left untouched if they
do not belong to an association.

In order to serialize these safely, we have to move this block of code
into embed_params!, which guards against infinite loops. This has the
added benefit of not doing unnecessary serialization when
send_only_modified_attributes is enabled.

lib/her/model/parse.rb

@@ -32,14 +32,7 @@ def parse(data)
 
         # @private
         def to_params(attributes, changes = {})
-          filtered_attributes = attributes.each_with_object({}) do |(key, value), memo|
-            case value
-            when ActiveModel::Serialization
-              value = value.serializable_hash.symbolize_keys
-            end
-
-            memo[key.to_sym] = value
-          end
+          filtered_attributes = attributes.symbolize_keys
 
           if her_api.options[:send_only_modified_attributes]
             filtered_attributes.slice! *changes.keys.map(&:to_sym)
@@ -93,6 +86,26 @@ def embed_params!(read_attributes, write_attributes)
 
             write_attributes[assoc[:data_key]] = value if value.present?
           end
+
+          write_attributes.each do |key, value|
+            value =
+              case value
+              when Her::Collection
+                value.map(&:to_params).reject(&:empty?)
+              when Her::Model
+                value.to_params
+              when ActiveModel::Serialization
+                value.serializable_hash.symbolize_keys
+              end
+
+            if value
+              if value.empty?
+                write_attributes.delete(key)
+              else
+                write_attributes[key] = value
+              end
+            end
+          end
         ensure
           Thread.current[:her_embedded_params_objects] = nil if first
         end

spec/model/parse_spec.rb

@@ -343,6 +343,10 @@ def to_params
           { fullname: "Lindsay Fünke" }
         end
       end
+
+      spawn_model "Foo::Comment" do
+        attributes :user
+      end
     end
 
     it "changes the request parameters for one-line resource creation" do
@@ -355,6 +359,12 @@ def to_params
       @user.save
       expect(@user.fullname).to eq("Lindsay Fünke")
     end
+
+    it 'changes the request parameters when nested inside an unassociated resource' do
+      @user = Foo::User.new(fullname: "Tobias Fünke")
+      @comment = Foo::Comment.new(user: @user)
+      expect(@comment.to_params).to eq(user: { fullname: "Lindsay Fünke" })
+    end
   end
 
   context "when parse_root_in_json set json_api to true" do