diff --git a/NEWS.md b/NEWS.md
index cf7cd544..f02fac5c 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -48,6 +48,15 @@ is now:
   * *PRs: [#989], [#964], [#917]*
   * *Original issue: [#867]*
 
+### Features
+
+* Add `required` and `optional` qualifiers to `belong_to` and `have_one`
+  matchers. (When using the `belong_to` matcher under Rails 5+, `required` is
+  assumed unless overridden.)
+
+  * *Original PR: [#956]*
+  * *Original issues: [#870], [#861]*
+
 [a6d09aa]: https://github.com/thoughtbot/shoulda-matchers/commit/a6d09aa5de0d546367e7b3d7177dfde6c66f7f05
 [#943]: https://github.com/thoughtbot/shoulda-matchers/pulls/943
 [#933]: https://github.com/thoughtbot/shoulda-matchers/issues/933
diff --git a/lib/shoulda/matchers/active_record.rb b/lib/shoulda/matchers/active_record.rb
index 7ed16585..4864b985 100644
--- a/lib/shoulda/matchers/active_record.rb
+++ b/lib/shoulda/matchers/active_record.rb
@@ -6,6 +6,8 @@ require "shoulda/matchers/active_record/association_matchers/join_table_matcher"
 require "shoulda/matchers/active_record/association_matchers/order_matcher"
 require "shoulda/matchers/active_record/association_matchers/through_matcher"
 require "shoulda/matchers/active_record/association_matchers/dependent_matcher"
+require "shoulda/matchers/active_record/association_matchers/required_matcher"
+require "shoulda/matchers/active_record/association_matchers/optional_matcher"
 require "shoulda/matchers/active_record/association_matchers/source_matcher"
 require "shoulda/matchers/active_record/association_matchers/model_reflector"
 require "shoulda/matchers/active_record/association_matchers/model_reflection"
diff --git a/lib/shoulda/matchers/active_record/association_matcher.rb b/lib/shoulda/matchers/active_record/association_matcher.rb
index 6489da90..fd470495 100644
--- a/lib/shoulda/matchers/active_record/association_matcher.rb
+++ b/lib/shoulda/matchers/active_record/association_matcher.rb
@@ -255,6 +255,48 @@ module Shoulda
       #
       # @return [AssociationMatcher]
       #
+      # ##### required
+      #
+      # Use `required` to assert that the association is not allowed to be nil.
+      # (Enabled by default in Rails 5+.)
+      #
+      #     class Person < ActiveRecord::Base
+      #       belongs_to :organization, required: true
+      #     end
+      #
+      #     # RSpec
+      #     describe Person
+      #       it { should belong_to(:organization).required }
+      #     end
+      #
+      #     # Minitest (Shoulda)
+      #     class PersonTest < ActiveSupport::TestCase
+      #       should belong_to(:organization).required
+      #     end
+      #
+      # @return [AssociationMatcher]
+      #
+      # ##### optional
+      #
+      # Use `optional` to assert that the association is allowed to be nil.
+      # (Rails 5+ only.)
+      #
+      #     class Person < ActiveRecord::Base
+      #       belongs_to :organization, optional: true
+      #     end
+      #
+      #     # RSpec
+      #     describe Person
+      #       it { should belong_to(:organization).optional }
+      #     end
+      #
+      #     # Minitest (Shoulda)
+      #     class PersonTest < ActiveSupport::TestCase
+      #       should belong_to(:organization).optional
+      #     end
+      #
+      # @return [AssociationMatcher]
+      #
       def belong_to(name)
         AssociationMatcher.new(:belongs_to, name)
       end
@@ -714,6 +756,25 @@ module Shoulda
       #       should have_one(:bank).autosave(true)
       #     end
       #
+      # ##### required
+      #
+      # Use `required` to assert that the association is not allowed to be nil.
+      # (Rails 5+ only.)
+      #
+      #     class Person < ActiveRecord::Base
+      #       has_one :brain, required: true
+      #     end
+      #
+      #     # RSpec
+      #     describe Person
+      #       it { should have_one(:brain).required }
+      #     end
+      #
+      #     # Minitest (Shoulda)
+      #     class PersonTest < ActiveSupport::TestCase
+      #       should have_one(:brain).required
+      #     end
+      #
       # @return [AssociationMatcher]
       #
       def have_one(name)
@@ -891,42 +952,67 @@ module Shoulda
           @options = {}
           @submatchers = []
           @missing = ''
+
+          if macro == :belongs_to
+            if RailsShim.active_record_gte_5?
+              required
+            else
+              optional
+            end
+          end
         end
 
         def through(through)
-          through_matcher = AssociationMatchers::ThroughMatcher.new(through, name)
-          add_submatcher(through_matcher)
+          add_submatcher(
+            AssociationMatchers::ThroughMatcher,
+            through,
+            name,
+          )
           self
         end
 
         def dependent(dependent)
-          dependent_matcher = AssociationMatchers::DependentMatcher.new(dependent, name)
-          add_submatcher(dependent_matcher)
+          add_submatcher(
+            AssociationMatchers::DependentMatcher,
+            dependent,
+            name,
+          )
           self
         end
 
         def order(order)
-          order_matcher = AssociationMatchers::OrderMatcher.new(order, name)
-          add_submatcher(order_matcher)
+          add_submatcher(
+            AssociationMatchers::OrderMatcher,
+            order,
+            name,
+          )
           self
         end
 
         def counter_cache(counter_cache = true)
-          counter_cache_matcher = AssociationMatchers::CounterCacheMatcher.new(counter_cache, name)
-          add_submatcher(counter_cache_matcher)
+          add_submatcher(
+            AssociationMatchers::CounterCacheMatcher,
+            counter_cache,
+            name,
+          )
           self
         end
 
         def inverse_of(inverse_of)
-          inverse_of_matcher =
-            AssociationMatchers::InverseOfMatcher.new(inverse_of, name)
-          add_submatcher(inverse_of_matcher)
+          add_submatcher(
+            AssociationMatchers::InverseOfMatcher,
+            inverse_of,
+            name,
+          )
           self
         end
 
         def source(source)
-          source_matcher = AssociationMatchers::SourceMatcher.new(source, name)
-          add_submatcher(source_matcher)
+          add_submatcher(
+            AssociationMatchers::SourceMatcher,
+            source,
+            name,
+          )
           self
         end
 
@@ -955,6 +1041,26 @@ module Shoulda
           self
         end
 
+        def required(required = true)
+          remove_submatcher(AssociationMatchers::OptionalMatcher)
+          add_submatcher(
+            AssociationMatchers::RequiredMatcher,
+            name,
+            required,
+          )
+          self
+        end
+
+        def optional(optional = true)
+          remove_submatcher(AssociationMatchers::RequiredMatcher)
+          add_submatcher(
+            AssociationMatchers::OptionalMatcher,
+            name,
+            optional,
+          )
+          self
+        end
+
         def validate(validate = true)
           @options[:validate] = validate
           self
@@ -1016,8 +1122,15 @@ module Shoulda
           @reflector ||= AssociationMatchers::ModelReflector.new(subject, name)
         end
 
-        def add_submatcher(matcher)
-          @submatchers << matcher
+        def add_submatcher(matcher_class, *args)
+          remove_submatcher(matcher_class)
+          submatchers << matcher_class.new(*args)
+        end
+
+        def remove_submatcher(matcher_class)
+          submatchers.delete_if do |submatcher|
+            submatcher.is_a?(matcher_class)
+          end
         end
 
         def macro_description
@@ -1039,7 +1152,7 @@ module Shoulda
 
         def missing_options
           missing_options = [missing, missing_options_for_failing_submatchers]
-          missing_options.flatten.compact.join(', ')
+          missing_options.flatten.select(&:present?).join(', ')
         end
 
         def failing_submatchers
diff --git a/lib/shoulda/matchers/active_record/association_matchers/optional_matcher.rb b/lib/shoulda/matchers/active_record/association_matchers/optional_matcher.rb
new file mode 100644
index 00000000..83bef57f
--- /dev/null
+++ b/lib/shoulda/matchers/active_record/association_matchers/optional_matcher.rb
@@ -0,0 +1,47 @@
+module Shoulda
+  module Matchers
+    module ActiveRecord
+      module AssociationMatchers
+        # @private
+        class OptionalMatcher
+          attr_reader :missing_option
+
+          def initialize(attribute_name, optional)
+            @attribute_name = attribute_name
+            @missing_option = ''
+            @submatcher = submatcher_class_for(optional).new(nil).
+              for(attribute_name).
+              with_message(:required)
+          end
+
+          def description
+            'required: true'
+          end
+
+          def matches?(subject)
+            if submatcher.matches?(subject)
+              true
+            else
+              @missing_option =
+                'the association should have been defined ' +
+                'with `optional: true`, but was not'
+              false
+            end
+          end
+
+          private
+
+          attr_reader :subject, :submatcher
+
+          def submatcher_class_for(optional)
+            if optional
+              ActiveModel::AllowValueMatcher
+            else
+              ActiveModel::DisallowValueMatcher
+            end
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/lib/shoulda/matchers/active_record/association_matchers/required_matcher.rb b/lib/shoulda/matchers/active_record/association_matchers/required_matcher.rb
new file mode 100644
index 00000000..54e29645
--- /dev/null
+++ b/lib/shoulda/matchers/active_record/association_matchers/required_matcher.rb
@@ -0,0 +1,50 @@
+module Shoulda
+  module Matchers
+    module ActiveRecord
+      module AssociationMatchers
+        # @private
+        class RequiredMatcher
+          attr_reader :missing_option
+
+          def initialize(attribute_name, required)
+            @missing_option = ''
+            @submatcher = submatcher_class_for(required).new(nil).
+              for(attribute_name).
+              with_message(validation_message_key)
+          end
+
+          def description
+            'required: true'
+          end
+
+          def matches?(subject)
+            if submatcher.matches?(subject)
+              true
+            else
+              @missing_option =
+                'the association should have been defined ' +
+                'with `required: true`, but was not'
+              false
+            end
+          end
+
+          private
+
+          attr_reader :subject, :submatcher
+
+          def submatcher_class_for(required)
+            if required
+              ActiveModel::DisallowValueMatcher
+            else
+              ActiveModel::AllowValueMatcher
+            end
+          end
+
+          def validation_message_key
+            RailsShim.validation_message_key_for_association_required_option
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/lib/shoulda/matchers/rails_shim.rb b/lib/shoulda/matchers/rails_shim.rb
index c5be6a30..c44decbd 100644
--- a/lib/shoulda/matchers/rails_shim.rb
+++ b/lib/shoulda/matchers/rails_shim.rb
@@ -21,8 +21,12 @@ module Shoulda
           Gem::Version.new('0')
         end
 
-        def active_record_major_version
-          ::ActiveRecord::VERSION::MAJOR
+        def active_record_gte_5?
+          Gem::Requirement.new('>= 5').satisfied_by?(active_record_version)
+        end
+
+        def active_record_version
+          Gem::Version.new(::ActiveRecord::VERSION::STRING)
         rescue NameError
           Gem::Version.new('0')
         end
@@ -92,7 +96,7 @@ module Shoulda
         end
 
         def tables_and_views(connection)
-          if active_record_major_version >= 5
+          if active_record_gte_5?
             connection.data_sources
           else
             connection.tables
@@ -107,6 +111,14 @@ module Shoulda
           end
         end
 
+        def validation_message_key_for_association_required_option
+          if active_record_gte_5?
+            :required
+          else
+            :blank
+          end
+        end
+
         private
 
         def simply_generate_validation_message(
diff --git a/spec/support/unit/helpers/active_record_versions.rb b/spec/support/unit/helpers/active_record_versions.rb
index cf66604b..1f4cb299 100644
--- a/spec/support/unit/helpers/active_record_versions.rb
+++ b/spec/support/unit/helpers/active_record_versions.rb
@@ -32,5 +32,9 @@ module UnitTests
     def active_record_uniqueness_supports_array_columns?
       active_record_version < 5
     end
+
+    def active_record_supports_required_for_associations?
+      active_record_version >= 5
+    end
   end
 end
diff --git a/spec/support/unit/matchers/fail_with_message_matcher.rb b/spec/support/unit/matchers/fail_with_message_matcher.rb
index 1a852356..56436f0c 100644
--- a/spec/support/unit/matchers/fail_with_message_matcher.rb
+++ b/spec/support/unit/matchers/fail_with_message_matcher.rb
@@ -24,13 +24,15 @@ module UnitTests
         lines << Shoulda::Matchers::Util.indent(expected, 2)
 
         if @actual
+          diff = differ.diff(@actual, expected)[1..-1]
+
           lines << 'Actually failed with:'
           lines << Shoulda::Matchers::Util.indent(@actual, 2)
-          lines << "Diff:"
-          lines << Shoulda::Matchers::Util.indent(
-            differ.diff(@actual, expected)[1..-1],
-            2
-          )
+
+          if diff
+            lines << 'Diff:'
+            lines << Shoulda::Matchers::Util.indent(diff, 2)
+          end
         else
           lines << 'However, the expectation did not fail at all.'
         end
diff --git a/spec/unit/shoulda/matchers/active_record/association_matcher_spec.rb b/spec/unit/shoulda/matchers/active_record/association_matcher_spec.rb
index f383487a..1a9d23f1 100644
--- a/spec/unit/shoulda/matchers/active_record/association_matcher_spec.rb
+++ b/spec/unit/shoulda/matchers/active_record/association_matcher_spec.rb
@@ -82,18 +82,18 @@ describe Shoulda::Matchers::ActiveRecord::AssociationMatcher, type: :model do
     end
 
     it 'accepts an association with a valid :inverse_of option' do
-      expect(belonging_to_parent(inverse_of: :children))
-        .to belong_to(:parent).inverse_of(:children)
+      expect(belonging_to_with_inverse(:parent, :children)).
+        to belong_to(:parent).inverse_of(:children)
     end
 
     it 'rejects an association with a bad :inverse_of option' do
-      expect(belonging_to_parent(inverse_of: :other_children))
-        .not_to belong_to(:parent).inverse_of(:children)
+      expect(belonging_to_with_inverse(:parent, :other_children)).
+        not_to belong_to(:parent).inverse_of(:children)
     end
 
     it 'rejects an association that has no :inverse_of option' do
-      expect(belonging_to_parent)
-        .not_to belong_to(:parent).inverse_of(:children)
+      expect(belonging_to_parent).
+        not_to belong_to(:parent).inverse_of(:children)
     end
 
     it 'accepts an association with a valid :conditions option' do
@@ -279,13 +279,92 @@ describe Shoulda::Matchers::ActiveRecord::AssociationMatcher, type: :model do
       end
     end
 
-    def belonging_to_parent(options = {})
-      define_model :parent
+    context 'given an association with neither :required nor :optional specified' do
+      if active_record_supports_required_for_associations?
+        it 'assumes it is required' do
+          expect(belonging_to_parent).to belong_to(:parent).required
+        end
+      else
+        it 'assumes it is optional' do
+          expect(belonging_to_parent).to belong_to(:parent).optional
+        end
+      end
+    end
+
+    context 'given an association with a matching :required option' do
+      it 'passes' do
+        expect(belonging_to_parent(required: true)).
+          to belong_to(:parent).required
+      end
+    end
+
+    context 'given an association with a non-matching :required option' do
+      it 'fails with an appropriate message' do
+        assertion = lambda do
+          expect(belonging_to_parent(required: false)).
+            to belong_to(:parent).required
+        end
+
+        message =
+          'Expected Child to have a belongs_to association called parent ' +
+          '(the association should have been defined with `required: true`, ' +
+          'but was not)'
+
+        expect(&assertion).to fail_with_message(message)
+      end
+    end
+
+    if active_record_supports_required_for_associations?
+      context 'given an association with a matching :optional option' do
+        it 'passes' do
+          expect(belonging_to_parent(optional: true)).
+            to belong_to(:parent).optional
+        end
+      end
+
+      context 'given an association with a non-matching :optional option' do
+        it 'fails with an appropriate message' do
+          assertion = lambda do
+            expect(belonging_to_parent(optional: false)).
+              to belong_to(:parent).optional
+          end
+
+          message =
+            'Expected Child to have a belongs_to association called parent ' +
+            '(the association should have been defined with `optional: ' +
+            'true`, but was not)'
+
+          expect(&assertion).to fail_with_message(message)
+        end
+      end
+    end
+
+    def belonging_to_parent(options = {}, parent_options = {})
+      define_model :parent, parent_options
       define_model :child, parent_id: :integer do
         belongs_to :parent, options
       end.new
     end
 
+    def belonging_to_with_inverse(association, inverse_association)
+      parent_model_name = association.to_s.singularize
+      child_model_name = inverse_association.to_s.singularize
+      parent_foreign_key = "#{parent_model_name}_id"
+
+      define_model parent_model_name do
+        has_many inverse_association
+      end
+
+      child_model = define_model(
+        child_model_name,
+        parent_foreign_key => :integer,
+      ) do
+        belongs_to association, inverse_of: inverse_association
+      end
+
+      child_model.new
+    end
+
     def belonging_to_non_existent_class(model_name, assoc_name, options = {})
       define_model model_name, "#{assoc_name}_id" => :integer do
         belongs_to assoc_name, options
@@ -826,6 +905,31 @@ describe Shoulda::Matchers::ActiveRecord::AssociationMatcher, type: :model do
       end
     end
 
+    if active_record_supports_required_for_associations?
+      context 'given an association with a matching :required option' do
+        it 'passes' do
+          expect(having_one_detail(required: true)).
+            to have_one(:detail).required
+        end
+      end
+    end
+
+    context 'given an association with a non-matching :required option' do
+      it 'fails with an appropriate message' do
+        assertion = lambda do
+          expect(having_one_detail(required: false)).
+            to have_one(:detail).required
+        end
+
+        message =
+          'Expected Person to have a has_one association called detail ' +
+          '(the association should have been defined with `required: true`, ' +
+          'but was not)'
+
+        expect(&assertion).to fail_with_message(message)
+      end
+    end
+
     def having_one_detail(options = {})
       define_model :detail, person_id: :integer
       define_model(:person).tap do |model|