diff --git a/nova/api/openstack/placement/objects/consumer.py b/nova/api/openstack/placement/objects/consumer.py
index a67eea7f36db..c9a7088e566b 100644
--- a/nova/api/openstack/placement/objects/consumer.py
+++ b/nova/api/openstack/placement/objects/consumer.py
@@ -217,6 +217,27 @@ class Consumer(base.VersionedObject, base.TimestampedObject):
         _create_in_db(self._context)
         self.obj_reset_changes()
 
+    def update(self):
+        """Used to update the consumer's project and user information without
+        incrementing the consumer's generation.
+        """
+        @db_api.placement_context_manager.writer
+        def _update_in_db(ctx):
+            upd_stmt = CONSUMER_TBL.update().values(
+                project_id=self.project.id, user_id=self.user.id)
+            # NOTE(jaypipes): We add the generation check to the WHERE clause
+            # above just for safety. We don't need to check that the statement
+            # actually updated a single row. If it did not, then the
+            # consumer.increment_generation() call that happens in
+            # AllocationList.create_all() will end up raising
+            # ConcurrentUpdateDetected anyway
+            upd_stmt = upd_stmt.where(sa.and_(
+                CONSUMER_TBL.c.id == self.id,
+                CONSUMER_TBL.c.generation == self.generation))
+            ctx.session.execute(upd_stmt)
+        _update_in_db(self._context)
+        self.obj_reset_changes()
+
     def increment_generation(self):
         """Increments the consumer's generation.
 
diff --git a/nova/api/openstack/placement/util.py b/nova/api/openstack/placement/util.py
index 095a16e27a9d..4192673e2d36 100644
--- a/nova/api/openstack/placement/util.py
+++ b/nova/api/openstack/placement/util.py
@@ -16,6 +16,7 @@ import re
 
 import jsonschema
 from oslo_config import cfg
+from oslo_log import log as logging
 from oslo_middleware import request_id
 from oslo_serialization import jsonutils
 from oslo_utils import timeutils
@@ -34,6 +35,7 @@ from nova.api.openstack.placement.objects import user as user_obj
 from nova.i18n import _
 
 CONF = cfg.CONF
+LOG = logging.getLogger(__name__)
 
 # Error code handling constants
 ENV_ERROR_CODE = 'placement.error_code'
@@ -586,6 +588,11 @@ def ensure_consumer(ctx, consumer_uuid, project_id, user_id,
     and User sub-objects and a boolean indicating whether a new Consumer object
     was created (as opposed to an existing consumer record retrieved)
 
+    :note: If the supplied project or user external identifiers do not match an
+           existing consumer's project and user identifiers, the existing
+           consumer's project and user IDs are updated to reflect the supplied
+           ones.
+
     :param ctx: The request context.
     :param consumer_uuid: The uuid of the consumer of the resources.
     :param project_id: The external ID of the project consuming the resources.
@@ -633,6 +640,37 @@ def ensure_consumer(ctx, consumer_uuid, project_id, user_id,
                           'expected_gen': consumer.generation,
                           'got_gen': consumer_generation,
                       })
+        # NOTE(jaypipes): The user may have specified a different project and
+        # user external ID than the one that we had for the consumer. If this
+        # is the case, go ahead and modify the consumer record with the
+        # newly-supplied project/user information, but do not bump the consumer
+        # generation (since it will be bumped in the
+        # AllocationList.create_all() method).
+        #
+        # TODO(jaypipes): This means that there may be a partial update.
+        # Imagine a scenario where a user calls POST /allocations, and the
+        # payload references two consumers. The first consumer is a new
+        # consumer and is auto-created. The second consumer is an existing
+        # consumer, but contains a different project or user ID than the
+        # existing consumer's record. If the eventual call to
+        # AllocationList.create_all() fails for whatever reason (say, a
+        # resource provider generation conflict or out of resources failure),
+        # we will end up deleting the auto-created consumer but we MAY not undo
+        # the changes to the second consumer's project and user ID. I say MAY
+        # and not WILL NOT because I'm not sure that the exception that gets
+        # raised from AllocationList.create_all() will cause the context
+        # manager's transaction to rollback automatically. I believe that the
+        # same transaction context is used for both util.ensure_consumer() and
+        # AllocationList.create_all() within the same HTTP request, but need to
+        # test this to be 100% certain...
+        if (project_id != consumer.project.external_id or
+                user_id != consumer.user.external_id):
+            LOG.debug("Supplied project or user ID for consumer %s was "
+                      "different than existing record. Updating consumer "
+                      "record.", consumer_uuid)
+            consumer.project = proj
+            consumer.user = user
+            consumer.update()
     except exception.NotFound:
         # If we are attempting to modify or create allocations after 1.26, we
         # need a consumer generation specified. The user must have specified
diff --git a/nova/tests/functional/api/openstack/placement/db/test_consumer.py b/nova/tests/functional/api/openstack/placement/db/test_consumer.py
index 907f44c3e45a..8fe167fc46d9 100644
--- a/nova/tests/functional/api/openstack/placement/db/test_consumer.py
+++ b/nova/tests/functional/api/openstack/placement/db/test_consumer.py
@@ -55,6 +55,35 @@ class ConsumerTestCase(tb.PlacementDbBaseTestCase):
         self.assertEqual(2, c.user.id)
         self.assertRaises(exception.ConsumerExists, c.create)
 
+    def test_update(self):
+        """Tests the scenario where a user supplies a different project/user ID
+        for an allocation's consumer and we call Consumer.update() to save that
+        information to the consumers table.
+        """
+        # First, create the consumer with the "fake-user" and "fake-project"
+        # user/project in the base test class's setUp
+        c = consumer_obj.Consumer(
+            self.ctx, uuid=uuids.consumer, user=self.user_obj,
+            project=self.project_obj)
+        c.create()
+        c = consumer_obj.Consumer.get_by_uuid(self.ctx, uuids.consumer)
+        self.assertEqual(self.project_obj.id, c.project.id)
+        self.assertEqual(self.user_obj.id, c.user.id)
+
+        # Now change the consumer's project and user to a different project
+        another_user = user_obj.User(self.ctx, external_id='another-user')
+        another_user.create()
+        another_proj = project_obj.Project(
+            self.ctx, external_id='another-project')
+        another_proj.create()
+
+        c.project = another_proj
+        c.user = another_user
+        c.update()
+        c = consumer_obj.Consumer.get_by_uuid(self.ctx, uuids.consumer)
+        self.assertEqual(another_proj.id, c.project.id)
+        self.assertEqual(another_user.id, c.user.id)
+
 
 @db_api.placement_context_manager.reader
 def _get_allocs_with_no_consumer_relationship(ctx):
diff --git a/nova/tests/functional/api/openstack/placement/gabbits/allocations-bug-1779717.yaml b/nova/tests/functional/api/openstack/placement/gabbits/allocations-bug-1779717.yaml
new file mode 100644
index 000000000000..13de8aa41dce
--- /dev/null
+++ b/nova/tests/functional/api/openstack/placement/gabbits/allocations-bug-1779717.yaml
@@ -0,0 +1,102 @@
+# Test that it's possible to change the project or user identifier for a
+# consumer by specifying a different project_id or user_id value in the payload
+# of both a PUT /allocations/{consumer_uuid} or POST /allocations
+
+fixtures:
+    - APIFixture
+
+defaults:
+    request_headers:
+        x-auth-token: admin
+        accept: application/json
+        content-type: application/json
+        openstack-api-version: placement 1.28
+
+tests:
+
+- name: create cn1
+  POST: /resource_providers
+  data:
+      name: cn1
+  status: 200
+
+- name: add inventory
+  PUT: $HISTORY['create cn1'].$RESPONSE['links[?rel = "inventories"].href']
+  data:
+    resource_provider_generation: 0
+    inventories:
+      VCPU:
+        total: 16
+      MEMORY_MB:
+        total: 2048
+
+- name: create allocations for consumer1
+  PUT: /allocations/11111111-1111-1111-1111-111111111111
+  data:
+    allocations:
+      $HISTORY['create cn1'].$RESPONSE['uuid']:
+        resources:
+          MEMORY_MB: 1024
+          VCPU: 2
+    project_id: $ENVIRON['PROJECT_ID']
+    user_id: $ENVIRON['USER_ID']
+    consumer_generation: null
+  status: 204
+
+- name: get allocations for consumer1
+  GET: /allocations/11111111-1111-1111-1111-111111111111
+  status: 200
+  response_json_paths:
+    $.project_id: $ENVIRON['PROJECT_ID']
+    $.user_id: $ENVIRON['USER_ID']
+
+- name: change the project for consumer1
+  PUT: /allocations/11111111-1111-1111-1111-111111111111
+  data:
+    allocations:
+      $HISTORY['create cn1'].$RESPONSE['uuid']:
+        resources:
+          MEMORY_MB: 1024
+          VCPU: 2
+    project_id: $ENVIRON['PROJECT_ID_ALT']
+    user_id: $ENVIRON['USER_ID']
+    consumer_generation: 1
+  status: 204
+
+- name: check consumer1's project is now the other project
+  GET: /allocations/11111111-1111-1111-1111-111111111111
+  status: 200
+  response_json_paths:
+    $.project_id: $ENVIRON['PROJECT_ID_ALT']
+    $.user_id: $ENVIRON['USER_ID']
+
+- name: create allocations for two consumers
+  POST: /allocations
+  data:
+    11111111-1111-1111-1111-111111111111:
+      allocations:
+        $HISTORY['create cn1'].$RESPONSE['uuid']:
+          resources:
+            MEMORY_MB: 1024
+            VCPU: 1
+      consumer_generation: 2
+      # Change consumer1's project back to the original PROJECT_ID
+      project_id: $ENVIRON['PROJECT_ID']
+      user_id: $ENVIRON['USER_ID']
+    22222222-2222-2222-2222-222222222222:
+      allocations:
+        $HISTORY['create cn1'].$RESPONSE['uuid']:
+          resources:
+            MEMORY_MB: 1024
+            VCPU: 1
+      consumer_generation: null
+      project_id: $ENVIRON['PROJECT_ID_ALT']
+      user_id: $ENVIRON['USER_ID_ALT']
+  status: 204
+
+- name: check consumer1's project is back to the original project
+  GET: /allocations/11111111-1111-1111-1111-111111111111
+  status: 200
+  response_json_paths:
+    $.project_id: $ENVIRON['PROJECT_ID']
+    $.user_id: $ENVIRON['USER_ID']