diff --git a/lib/jsonapi_compliable/adapters/active_record_sideloading.rb b/lib/jsonapi_compliable/adapters/active_record_sideloading.rb
index 3804f31..b2ffbf5 100644
--- a/lib/jsonapi_compliable/adapters/active_record_sideloading.rb
+++ b/lib/jsonapi_compliable/adapters/active_record_sideloading.rb
@@ -47,7 +47,7 @@ def belongs_to(association_name, scope: nil, resource:, foreign_key:, primary_ke
       def has_one(association_name, scope: nil, resource:, foreign_key:, primary_key: :id, &blk)
         _scope = scope
 
-        allow_sideload association_name, resource: resource do
+        allow_sideload association_name, type: :has_one, foreign_key: foreign_key, primary_key: primary_key, resource: resource do
           scope do |parents|
             parent_ids = parents.map { |p| p.send(primary_key) }
             _scope.call.where(foreign_key => parent_ids.uniq.compact)
@@ -71,7 +71,7 @@ def has_and_belongs_to_many(association_name, scope: nil, resource:, foreign_key
         fk      = foreign_key.values.first
         _scope  = scope
 
-        allow_sideload association_name, resource: resource do
+        allow_sideload association_name, type: :habtm, foreign_key: foreign_key, primary_key: primary_key, resource: resource do
           scope do |parents|
             parent_ids = parents.map { |p| p.send(primary_key) }
             parent_ids.uniq!
@@ -94,14 +94,14 @@ def has_and_belongs_to_many(association_name, scope: nil, resource:, foreign_key
       end
 
       def polymorphic_belongs_to(association_name, group_by:, groups:, &blk)
-        allow_sideload association_name, polymorphic: true do
-          group_by(&group_by)
+        allow_sideload association_name, type: :polymorphic_belongs_to, polymorphic: true do
+          group_by(group_by)
 
           groups.each_pair do |type, config|
             primary_key = config[:primary_key] || :id
             foreign_key = config[:foreign_key]
 
-            allow_sideload type, resource: config[:resource] do
+            allow_sideload type, parent: self, primary_key: primary_key, foreign_key: foreign_key, type: :belongs_to, resource: config[:resource] do
               scope do |parents|
                 parent_ids = parents.map { |p| p.send(foreign_key) }
                 parent_ids.compact!
diff --git a/lib/jsonapi_compliable/sideload.rb b/lib/jsonapi_compliable/sideload.rb
index fb32829..a5db152 100644
--- a/lib/jsonapi_compliable/sideload.rb
+++ b/lib/jsonapi_compliable/sideload.rb
@@ -6,7 +6,7 @@ module JsonapiCompliable
   # @attr_reader [Hash] sideloads The associated sibling sideloads
   # @attr_reader [Proc] scope_proc The configured 'scope' block
   # @attr_reader [Proc] assign_proc The configured 'assign' block
-  # @attr_reader [Proc] grouper The configured 'group_by' proc
+  # @attr_reader [Symbol] grouping_field The configured 'group_by' symbol
   # @attr_reader [Symbol] foreign_key The attribute used to match objects - need not be a true database foreign key.
   # @attr_reader [Symbol] primary_key The attribute used to match objects - need not be a true database primary key.
   # @attr_reader [Symbol] type One of :has_many, :belongs_to, etc
@@ -15,10 +15,11 @@ class Sideload
       :resource_class,
       :polymorphic,
       :polymorphic_groups,
+      :parent,
       :sideloads,
       :scope_proc,
       :assign_proc,
-      :grouper,
+      :grouping_field,
       :foreign_key,
       :primary_key,
       :type
@@ -28,12 +29,13 @@ class Sideload
     # An anonymous Resource will be assigned when none provided.
     #
     # @see Adapters::Abstract#sideloading_module
-    def initialize(name, type: nil, resource: nil, polymorphic: false, primary_key: :id, foreign_key: nil)
+    def initialize(name, type: nil, resource: nil, polymorphic: false, primary_key: :id, foreign_key: nil, parent: nil)
       @name               = name
       @resource_class     = (resource || Class.new(Resource))
       @sideloads          = {}
       @polymorphic        = !!polymorphic
       @polymorphic_groups = {} if polymorphic?
+      @parent             = parent
       @primary_key        = primary_key
       @foreign_key        = foreign_key
       @type               = type
@@ -55,7 +57,7 @@ def resource
     # +Business+ or +Government+:
     #
     #   allow_sideload :organization, :polymorphic: true do
-    #     group_by { |record| record.organization_type }
+    #     group_by :organization_type
     #
     #     allow_sideload 'Business', resource: BusinessResource do
     #       # ... code ...
@@ -70,7 +72,7 @@ def resource
     # with ActiveRecord:
     #
     #   polymorphic_belongs_to :organization,
-    #     group_by: ->(office) { office.organization_type },
+    #     group_by: :organization_type,
     #     groups: {
     #       'Business' => {
     #         scope: -> { Business.all },
@@ -181,21 +183,25 @@ def assign(&blk)
     # @see #name
     # @see #type
     def associate(parent, child)
-      resource_class.config[:adapter].associate(parent, child, name, type)
+      association_name = @parent ? @parent.name : name
+      resource_class.config[:adapter].associate parent,
+        child,
+        association_name,
+        type
     end
 
-    # Define a proc that groups the parent records. For instance, with
+    # Define an attribute that groups the parent records. For instance, with
     # an ActiveRecord polymorphic belongs_to there will be a +parent_id+
     # and +parent_type+. We would want to group on +parent_type+:
     #
     #  allow_sideload :organization, polymorphic: true do
     #    # group parent_type, parent here is 'organization'
-    #    group_by ->(office) { office.organization_type }
+    #    group_by :organization_type
     #  end
     #
     # @see #polymorphic?
-    def group_by(&grouper)
-      @grouper = grouper
+    def group_by(grouping_field)
+      @grouping_field = grouping_field
     end
 
     # Resolve the sideload.
@@ -323,6 +329,13 @@ def to_hash(processed = [])
       result
     end
 
+    # @api private
+    def polymorphic_child_for_type(type)
+      polymorphic_groups.values.find do |v|
+        v.resource_class.config[:type] == type
+      end
+    end
+
     private
 
     def nested_sideload_hash(sideload, processed)
@@ -333,8 +346,24 @@ def nested_sideload_hash(sideload, processed)
       end
     end
 
+    def polymorphic_grouper(grouping_field)
+      lambda do |record|
+        if record.is_a?(Hash)
+          if record.keys[0].is_a?(Symbol)
+            record[grouping_field]
+          else
+            record[grouping_field.to_s]
+          end
+        else
+          record.send(grouping_field)
+        end
+      end
+    end
+
     def resolve_polymorphic(parents, query)
-      parents.group_by(&@grouper).each_pair do |group_type, group_members|
+      grouper = polymorphic_grouper(@grouping_field)
+
+      parents.group_by(&grouper).each_pair do |group_type, group_members|
         sideload_for_group = @polymorphic_groups[group_type]
         if sideload_for_group
           sideload_for_group.resolve(group_members, query, name)
diff --git a/lib/jsonapi_compliable/util/persistence.rb b/lib/jsonapi_compliable/util/persistence.rb
index b731618..a81ccff 100644
--- a/lib/jsonapi_compliable/util/persistence.rb
+++ b/lib/jsonapi_compliable/util/persistence.rb
@@ -52,11 +52,18 @@ def run
   def update_foreign_key(parent_object, attrs, x)
     if [:destroy, :disassociate].include?(x[:meta][:method])
       attrs[x[:foreign_key]] = nil
+      update_foreign_type(attrs, x, null: true) if x[:is_polymorphic]
     else
       attrs[x[:foreign_key]] = parent_object.send(x[:primary_key])
+      update_foreign_type(attrs, x) if x[:is_polymorphic]
     end
   end
 
+  def update_foreign_type(attrs, x, null: false)
+    grouping_field = x[:sideload].parent.grouping_field
+    attrs[grouping_field] = null ? nil : x[:sideload].name
+  end
+
   def update_foreign_key_for_parents(parents)
     parents.each do |x|
       update_foreign_key(x[:object], @attributes, x)
@@ -88,7 +95,7 @@ def persist_object(method, attributes)
 
   def process_has_many(relationships)
     [].tap do |processed|
-      iterate(except: [:belongs_to]) do |x|
+      iterate(except: [:polymorphic_belongs_to, :belongs_to]) do |x|
         yield x
         x[:object] = x[:sideload].resource
           .persist_with_relationships(x[:meta], x[:attributes], x[:relationships])
@@ -99,7 +106,7 @@ def process_has_many(relationships)
 
   def process_belongs_to(relationships)
     [].tap do |processed|
-      iterate(only: [:belongs_to]) do |x|
+      iterate(only: [:polymorphic_belongs_to, :belongs_to]) do |x|
         x[:object] = x[:sideload].resource
           .persist_with_relationships(x[:meta], x[:attributes], x[:relationships])
         processed << x
diff --git a/lib/jsonapi_compliable/util/relationship_payload.rb b/lib/jsonapi_compliable/util/relationship_payload.rb
index b581921..ed30db7 100644
--- a/lib/jsonapi_compliable/util/relationship_payload.rb
+++ b/lib/jsonapi_compliable/util/relationship_payload.rb
@@ -43,8 +43,14 @@ def should_yield?(type)
       end
 
       def payload_for(sideload, relationship_payload)
+        if sideload.polymorphic?
+          type     = relationship_payload[:meta][:jsonapi_type]
+          sideload = sideload.polymorphic_child_for_type(type)
+        end
+
         {
           sideload: sideload,
+          is_polymorphic: !sideload.parent.nil?,
           primary_key: sideload.primary_key,
           foreign_key: sideload.foreign_key,
           attributes: relationship_payload[:attributes],
diff --git a/lib/jsonapi_compliable/util/validation_response.rb b/lib/jsonapi_compliable/util/validation_response.rb
index 6670970..7085774 100644
--- a/lib/jsonapi_compliable/util/validation_response.rb
+++ b/lib/jsonapi_compliable/util/validation_response.rb
@@ -33,7 +33,8 @@ def to_a
   private
 
   def valid_object?(object)
-    object.respond_to?(:errors) && object.errors.blank?
+    !object.respond_to?(:errors) ||
+      (object.respond_to?(:errors) && object.errors.blank?)
   end
 
   def all_valid?(model, deserialized_params)
diff --git a/spec/fixtures/employee_directory.rb b/spec/fixtures/employee_directory.rb
index b3d7edb..224dece 100644
--- a/spec/fixtures/employee_directory.rb
+++ b/spec/fixtures/employee_directory.rb
@@ -3,7 +3,17 @@
     t.string :description
   end
 
+  create_table :offices do |t|
+    t.string :address
+  end
+
+  create_table :home_offices do |t|
+    t.string :address
+  end
+
   create_table :employees do |t|
+    t.string :workspace_type
+    t.integer :workspace_id
     t.integer :classification_id
     t.string :first_name
     t.string :last_name
@@ -30,7 +40,16 @@ class Classification < ApplicationRecord
   validates :description, presence: true
 end
 
+class Office < ApplicationRecord
+  has_many :employees, as: :workspace
+end
+
+class HomeOffice < ApplicationRecord
+  has_many :employees, as: :workspace
+end
+
 class Employee < ApplicationRecord
+  belongs_to :workspace, polymorphic: true
   belongs_to :classification
   has_many :positions
   validates :first_name, presence: true
@@ -69,6 +88,16 @@ class PositionResource < ApplicationResource
     resource: DepartmentResource
 end
 
+class OfficeResource < ApplicationResource
+  type 'offices'
+  model Office
+end
+
+class HomeOfficeResource < ApplicationResource
+  type 'home_offices'
+  model HomeOffice
+end
+
 class EmployeeResource < ApplicationResource
   type 'employees'
   model Employee
@@ -81,6 +110,20 @@ class EmployeeResource < ApplicationResource
     scope: -> { Position.all },
     foreign_key: :employee_id,
     resource: PositionResource
+  polymorphic_belongs_to :workspace,
+    group_by: :workspace_type,
+    groups: {
+      'Office' => {
+        scope: -> { Office.all },
+        resource: OfficeResource,
+        foreign_key: :workspace_id
+      },
+      'HomeOffice' => {
+        scope: -> { HomeOffice.all },
+        resource: HomeOfficeResource,
+        foreign_key: :workspace_id
+      }
+    }
 end
 
 class SerializableAbstract < JSONAPI::Serializable::Resource
diff --git a/spec/integration/rails/finders_spec.rb b/spec/integration/rails/finders_spec.rb
index bc3303f..b4d5326 100644
--- a/spec/integration/rails/finders_spec.rb
+++ b/spec/integration/rails/finders_spec.rb
@@ -86,7 +86,7 @@ class AuthorResource < JsonapiCompliable::Resource
           foreign_key: { author_hobbies: :author_id }
 
         polymorphic_belongs_to :dwelling,
-          group_by: proc { |author| author.dwelling_type },
+          group_by: :dwelling_type,
           groups: {
             'House' => {
               foreign_key: :dwelling_id,
diff --git a/spec/integration/rails/persistence_spec.rb b/spec/integration/rails/persistence_spec.rb
index 205097c..899745d 100644
--- a/spec/integration/rails/persistence_spec.rb
+++ b/spec/integration/rails/persistence_spec.rb
@@ -149,6 +149,62 @@ def do_put(id)
       end
     end
 
+    describe 'nested polymorphic relationship' do
+      let(:workspace_type) { 'offices' }
+
+      let(:payload) do
+        {
+          data: {
+            type: 'employees',
+            attributes: { first_name: 'Joe' },
+            relationships: {
+              workspace: {
+                data: {
+                  :'temp-id' => 'work1', type: workspace_type, method: 'create'
+                }
+              }
+            }
+          },
+          included: [
+            {
+              type: workspace_type,
+              :'temp-id' => 'work1',
+              attributes: {
+                address: 'Fake Workspace Address'
+              }
+            }
+          ]
+        }
+      end
+
+      context 'with jsonapi type "offices"' do
+        it 'associates workspace as office' do
+          do_post
+          employee = Employee.first
+          expect(employee.workspace).to be_a(Office)
+        end
+      end
+
+      context 'with jsonapi type "home_offices"' do
+        let(:workspace_type) { 'home_offices' }
+
+        it 'associates workspace as home office' do
+          do_post
+          employee = Employee.first
+          expect(employee.workspace).to be_a(HomeOffice)
+        end
+      end
+
+      it 'saves the relationship correctly' do
+        expect {
+          do_post
+        }.to change { Employee.count }.by(1)
+        employee = Employee.first
+        workspace = employee.workspace
+        expect(workspace.address).to eq('Fake Workspace Address')
+      end
+    end
+
     describe 'nested create' do
       let(:payload) do
         {
diff --git a/spec/sideload_spec.rb b/spec/sideload_spec.rb
index 95c6ca3..dfe28b9 100644
--- a/spec/sideload_spec.rb
+++ b/spec/sideload_spec.rb
@@ -53,7 +53,7 @@ def foo
       end
 
       before do
-        instance.group_by { |parent| parent[:type] }
+        instance.group_by :type
 
         instance.allow_sideload 'foo', resource: foo_resource do
           scope { |parents| [{ parent_id: 1 }] }
@@ -147,6 +147,30 @@ def foo
     end
   end
 
+  describe '#associate' do
+    before do
+      instance.instance_variable_set(:@type, :has_many)
+    end
+
+    it 'delegates to adapter' do
+      expect(instance.resource_class.config[:adapter])
+        .to receive(:associate).with('parent', 'child', :foo, :has_many)
+      instance.associate('parent', 'child')
+    end
+
+    context 'when a polymorphic child' do
+      before do
+        instance.instance_variable_set(:@parent, double(name: :parent_name))
+      end
+
+      it 'passes parent name as association name' do
+        expect(instance.resource_class.config[:adapter])
+          .to receive(:associate).with('parent', 'child', :parent_name, :has_many)
+        instance.associate('parent', 'child')
+      end
+    end
+  end
+
   describe '#to_hash' do
     # Set this up to catch any recursive trap
     let(:resource) do
diff --git a/spec/sideloading_spec.rb b/spec/sideloading_spec.rb
index 5586a13..8f65d52 100644
--- a/spec/sideloading_spec.rb
+++ b/spec/sideloading_spec.rb
@@ -95,7 +95,7 @@
 
     _dwelling_resource = dwelling_resource
     resource_class.allow_sideload :dwelling, polymorphic: true do
-      group_by { |author| binding.pry;author[:dwelling_type] }
+      group_by :dwelling_type
 
       allow_sideload 'House', resource: _dwelling_resource do
         scope do |authors|
@@ -201,7 +201,7 @@ def json
 
       _dwelling_resource = dwelling_resource
       resource_class.allow_sideload :dwelling, polymorphic: true do
-        group_by { |author| author[:dwelling_type] }
+        group_by :dwelling_type
 
         allow_sideload 'House', resource: _dwelling_resource do
           scope do |authors|