diff --git a/src/Access/RelationshipFieldAccess.php b/src/Access/RelationshipFieldAccess.php
index eefb7f0..0be984c 100644
--- a/src/Access/RelationshipFieldAccess.php
+++ b/src/Access/RelationshipFieldAccess.php
@@ -41,8 +41,8 @@ class RelationshipFieldAccess implements AccessInterface {
    */
   public function access(Request $request, Route $route, AccountInterface $account) {
     $relationship_field_name = $route->getRequirement(static::ROUTE_REQUIREMENT_KEY);
-    $field_operation = $request->isMethodSafe() ? 'view' : 'edit';
-    $entity_operation = $request->isMethodSafe() ? 'view' : 'update';
+    $field_operation = $request->isMethodCacheable() ? 'view' : 'edit';
+    $entity_operation = $request->isMethodCacheable() ? 'view' : 'update';
     if ($resource_type = $request->get(Routes::RESOURCE_TYPE_KEY)) {
       $entity = $request->get($resource_type->getEntityTypeId());
       if ($entity instanceof FieldableEntityInterface && $entity->hasField($relationship_field_name)) {
@@ -50,7 +50,11 @@ class RelationshipFieldAccess implements AccessInterface {
         $field_access = $entity->get($relationship_field_name)->access($field_operation, $account, TRUE);
         $access_result = $entity_access->andIf($field_access);
         if ($access_result instanceof AccessResultReasonInterface) {
-          $access_result->setReason("The current user is not allowed to $field_operation this relationship. " . $access_result->getReason());
+          $reason = "The current user is not allowed to {$field_operation} this relationship.";
+          if ($access_reason = $access_result->getReason()) {
+            $reason .= $access_reason;
+          };
+          $access_result->setReason($reason);
         }
         return $access_result;
       }
diff --git a/src/Controller/EntityResource.php b/src/Controller/EntityResource.php
index 2d6235b..f171f8e 100644
--- a/src/Controller/EntityResource.php
+++ b/src/Controller/EntityResource.php
@@ -844,9 +844,6 @@ class EntityResource {
       // @todo Is this really the right path?
       throw new EntityAccessDeniedHttpException($entity, $combined_access, $related_field, "The current user is not allowed to $operation this relationship.");
     }
-    if (!($field_list = $entity->get($related_field)) || !$this->isRelationshipField($field_list)) {
-      throw new NotFoundHttpException(sprintf('The relationship %s is not present in this resource.', $related_field));
-    }
   }
 
   /**
@@ -930,25 +927,6 @@ class EntityResource {
     throw new EntityAccessDeniedHttpException($original_field->getEntity(), $field_edit_access, '/data/attributes/' . $field_name, sprintf('The current user is not allowed to PATCH the selected field (%s).', $field_name));
   }
 
-  /**
-   * Checks if is a relationship field.
-   *
-   * @param \Drupal\Core\Field\FieldItemListInterface $entity_field
-   *   Entity field.
-   *
-   * @return bool
-   *   Returns TRUE if entity field is a relationship field with non-internal
-   *   target resource types, FALSE otherwise.
-   */
-  protected function isRelationshipField(FieldItemListInterface $entity_field) {
-    $resource_types = $this->resourceType->getRelatableResourceTypesByField(
-      $this->resourceType->getInternalName($entity_field->getName())
-    );
-    return !empty($resource_types) && array_reduce($resource_types, function ($has_external, $resource_type) {
-      return $has_external ? TRUE : !$resource_type->isInternal();
-    }, FALSE);
-  }
-
   /**
    * Build a collection of the entities to respond with and access objects.
    *
diff --git a/src/Routing/Routes.php b/src/Routing/Routes.php
index 2412f4d..764a35b 100644
--- a/src/Routing/Routes.php
+++ b/src/Routing/Routes.php
@@ -231,7 +231,13 @@ class Routes implements ContainerInjectionInterface {
       $routes->add(static::getRouteName($resource_type, 'individual.delete'), $individual_remove_route);
     }
 
-    foreach ($resource_type->getRelatableResourceTypes() as $relationship_field_name => $target_resource_type) {
+    foreach ($resource_type->getRelatableResourceTypes() as $relationship_field_name => $target_resource_types) {
+      // Do not create routes for relationships that only target internal
+      // resource types.
+      if (!static::hasNonInternalTargetResourceTypes($target_resource_types)) {
+        continue;
+      }
+
       // Get an individual resource's related resources.
       $related_route = new Route("/{$path}/{{$entity_type_id}}/{$relationship_field_name}");
       $related_route->setMethods(['GET']);
@@ -313,6 +319,22 @@ class Routes implements ContainerInjectionInterface {
     return sprintf('jsonapi.%s.%s', $resource_type->getTypeName(), $route_type);
   }
 
+  /**
+   * Determines if an array of resource types has any non-internal ones.
+   *
+   * @param \Drupal\jsonapi\ResourceType\ResourceType[] $resource_types
+   *   The resource types to check.
+   *
+   * @return bool
+   *   TRUE if there is at least one non-internal resource type in the given
+   *   array; FALSE otherwise.
+   */
+  protected static function hasNonInternalTargetResourceTypes(array $resource_types) {
+    return array_reduce($resource_types, function ($carry, ResourceType $target) {
+      return $carry || !$target->isInternal();
+    }, FALSE);
+  }
+
   /**
    * Gets the resource type from a route or request's parameters.
    *
diff --git a/tests/src/Unit/Routing/RoutesTest.php b/tests/src/Unit/Routing/RoutesTest.php
index ba678dd..726bd08 100644
--- a/tests/src/Unit/Routing/RoutesTest.php
+++ b/tests/src/Unit/Routing/RoutesTest.php
@@ -66,9 +66,15 @@ class RoutesTest extends UnitTestCase {
     // Get the route collection and start making assertions.
     $routes = $this->routes['ok']->routes();
 
-    // Make sure that there are 7 routes for the non-internal resource and 1 for
-    // the entry point.
-    $this->assertEquals(8, $routes->count());
+    // - 2 collection routes; GET & POST for the non-internal resource type.
+    // - 3 individual routes; GET, PATCH & DELETE for the non-internal resource
+    //   type.
+    // - 2 related routes; GET for the non-internal resource type relationships
+    //   fields: external & both.
+    // - 2 relationship routes; all HTTP methods handled by the one route for
+    //   the non-internal resource type.
+    // - 1 for the JSON API entry point.
+    $this->assertEquals(10, $routes->count());
 
     $iterator = $routes->getIterator();
     // Check the collection route.
@@ -142,8 +148,8 @@ class RoutesTest extends UnitTestCase {
 
     // Check the related route.
     /** @var \Symfony\Component\Routing\Route $route */
-    $route = $iterator->offsetGet('jsonapi.entity_type_1--bundle_1_1.related');
-    $this->assertSame('/jsonapi/entity_type_1/bundle_1_1/{entity_type_1}/{related}', $route->getPath());
+    $route = $iterator->offsetGet('jsonapi.entity_type_1--bundle_1_1.external.related');
+    $this->assertSame('/jsonapi/entity_type_1/bundle_1_1/{entity_type_1}/external', $route->getPath());
     $this->assertSame('entity_type_1--bundle_1_1', $route->getDefault(Routes::RESOURCE_TYPE_KEY));
     $this->assertEquals(['GET'], $route->getMethods());
     $this->assertSame(Routes::FRONT_CONTROLLER, $route->getDefault(RouteObjectInterface::CONTROLLER_NAME));
@@ -163,8 +169,8 @@ class RoutesTest extends UnitTestCase {
 
     // Check the relationships route.
     /** @var \Symfony\Component\Routing\Route $route */
-    $route = $iterator->offsetGet('jsonapi.entity_type_1--bundle_1_1.relationship');
-    $this->assertSame('/jsonapi/entity_type_1/bundle_1_1/{entity_type_1}/relationships/{related}', $route->getPath());
+    $route = $iterator->offsetGet('jsonapi.entity_type_1--bundle_1_1.both.relationship');
+    $this->assertSame('/jsonapi/entity_type_1/bundle_1_1/{entity_type_1}/relationships/both', $route->getPath());
     $this->assertSame('entity_type_1--bundle_1_1', $route->getDefault(Routes::RESOURCE_TYPE_KEY));
     $this->assertEquals(['GET', 'POST', 'PATCH', 'DELETE'], $route->getMethods());
     $this->assertSame(Routes::FRONT_CONTROLLER, $route->getDefault(RouteObjectInterface::CONTROLLER_NAME));
@@ -192,8 +198,10 @@ class RoutesTest extends UnitTestCase {
     return [
       ['jsonapi.entity_type_1--bundle_1_1.individual'],
       ['jsonapi.entity_type_1--bundle_1_1.collection'],
-      ['jsonapi.entity_type_1--bundle_1_1.related'],
-      ['jsonapi.entity_type_1--bundle_1_1.relationship'],
+      ['jsonapi.entity_type_1--bundle_1_1.external.related'],
+      ['jsonapi.entity_type_1--bundle_1_1.external.relationship'],
+      ['jsonapi.entity_type_1--bundle_1_1.both.related'],
+      ['jsonapi.entity_type_1--bundle_1_1.both.relationship'],
       ['jsonapi.resource_list'],
     ];
   }
@@ -215,8 +223,8 @@ class RoutesTest extends UnitTestCase {
       ['jsonapi.entity_type_2--bundle_2_1.individual'],
       ['jsonapi.entity_type_2--bundle_2_1.collection'],
       ['jsonapi.entity_type_2--bundle_2_1.collection.post'],
-      ['jsonapi.entity_type_2--bundle_2_1.related'],
-      ['jsonapi.entity_type_2--bundle_2_1.relationship'],
+      ['jsonapi.entity_type_2--bundle_2_1.internal.related'],
+      ['jsonapi.entity_type_2--bundle_2_1.internal.relationship'],
     ];
   }