db: Replace use of backref

Per the SQLAlchemy docs [1]:

  The relationship.backref keyword should be considered legacy, and use
  of relationship.back_populates with explicit relationship() constructs
  should be preferred.

A number of the relationships defined here don't have foreign keys (long
live mordred?) so their conversion is slightly more difficult than would
otherwise be the case. A blog post is available to explain what's going
on [2] and might be worth a read. The learnings from that blog post do
have the benefit of allowing us to simplify some existing relationships
that had unnecessary arguments defined.

[1] https://docs.sqlalchemy.org/en/14/orm/backref.html
[2] https://that.guru/blog/sqlalchemy-relationships-without-foreign-keys/

Change-Id: I5a135b012dabdff7cf06204fc3c5438aaa0985c9
Signed-off-by: Stephen Finucane <sfinucan@redhat.com>

nova/db/api/models.py

@@ -147,11 +147,15 @@ class CellMapping(BASE):
     transport_url = sa.Column(sa.Text())
     database_connection = sa.Column(sa.Text())
     disabled = sa.Column(sa.Boolean, default=False)
+
     host_mapping = orm.relationship(
         'HostMapping',
-        backref=orm.backref('cell_mapping', uselist=False),
+        back_populates='cell_mapping',
-        foreign_keys=id,
+    )
-        primaryjoin='CellMapping.id == HostMapping.cell_id')
+    instance_mapping = orm.relationship(
+        'InstanceMapping',
+        back_populates='cell_mapping',
+    )
 
 
 class InstanceMapping(BASE):
@@ -178,11 +182,11 @@ class InstanceMapping(BASE):
     # transition period first.
     user_id = sa.Column(sa.String(255), nullable=True)
     queued_for_delete = sa.Column(sa.Boolean)
+
     cell_mapping = orm.relationship(
         'CellMapping',
-        backref=orm.backref('instance_mapping', uselist=False),
+        back_populates='instance_mapping',
-        foreign_keys=cell_id,
+    )
-        primaryjoin='InstanceMapping.cell_id == CellMapping.id')
 
 
 class HostMapping(BASE):
@@ -198,6 +202,11 @@ class HostMapping(BASE):
         sa.Integer, sa.ForeignKey('cell_mappings.id'), nullable=False)
     host = sa.Column(sa.String(255), nullable=False)
 
+    cell_mapping = orm.relationship(
+        'CellMapping',
+        back_populates='host_mapping',
+    )
+
 
 class RequestSpec(BASE):
     """Represents the information passed to the scheduler."""
@@ -235,6 +244,9 @@ class Flavors(BASE):
     is_public = sa.Column(sa.Boolean, default=True)
     description = sa.Column(sa.Text)
 
+    extra_specs = orm.relationship('FlavorExtraSpecs', back_populates='flavor')
+    projects = orm.relationship('FlavorProjects', back_populates='flavor')
+
 
 class FlavorExtraSpecs(BASE):
     """Represents additional specs as key/value pairs for a flavor"""
@@ -251,10 +263,8 @@ class FlavorExtraSpecs(BASE):
     value = sa.Column(sa.String(255))
     flavor_id = sa.Column(
         sa.Integer, sa.ForeignKey('flavors.id'), nullable=False)
-    flavor = orm.relationship(
+
-        Flavors, backref='extra_specs',
+    flavor = orm.relationship(Flavors, back_populates='extra_specs')
-        foreign_keys=flavor_id,
-        primaryjoin='FlavorExtraSpecs.flavor_id == Flavors.id')
 
 
 class FlavorProjects(BASE):
@@ -267,10 +277,8 @@ class FlavorProjects(BASE):
     flavor_id = sa.Column(
         sa.Integer, sa.ForeignKey('flavors.id'), nullable=False)
     project_id = sa.Column(sa.String(255), nullable=False)
-    flavor = orm.relationship(
+
-        Flavors, backref='projects',
+    flavor = orm.relationship(Flavors, back_populates='projects')
-        foreign_keys=flavor_id,
-        primaryjoin='FlavorProjects.flavor_id == Flavors.id')
 
 
 class BuildRequest(BASE):
@@ -354,12 +362,9 @@ class InstanceGroup(BASE):
     project_id = sa.Column(sa.String(255))
     uuid = sa.Column(sa.String(36), nullable=False)
     name = sa.Column(sa.String(255))
-    _policies = orm.relationship(
+
-        InstanceGroupPolicy,
+    _policies = orm.relationship(InstanceGroupPolicy)
-        primaryjoin='InstanceGroup.id == InstanceGroupPolicy.group_id')
+    _members = orm.relationship(InstanceGroupMember)
-    _members = orm.relationship(
-        InstanceGroupMember,
-        primaryjoin='InstanceGroup.id == InstanceGroupMember.group_id')
 
     @property
     def policy(self):
@@ -487,7 +492,5 @@ class Reservation(BASE):
     resource = sa.Column(sa.String(255))
     delta = sa.Column(sa.Integer, nullable=False)
     expire = sa.Column(sa.DateTime)
-    usage = orm.relationship(
+
-        "QuotaUsage",
+    usage = orm.relationship('QuotaUsage')
-        foreign_keys=usage_id,
-        primaryjoin='Reservation.usage_id == QuotaUsage.id')

nova/db/main/models.py

@@ -161,8 +161,8 @@ class Service(BASE, NovaBase, models.SoftDeleteMixin):
     version = sa.Column(sa.Integer, default=0)
 
     instance = orm.relationship(
-        "Instance",
+        'Instance',
-        backref='services',
+        back_populates='services',
         primaryjoin='and_(Service.host == Instance.host,'
                     'Service.binary == "nova-compute",'
                     'Instance.deleted == 0)',
@@ -425,6 +425,96 @@ class Instance(BASE, NovaBase, models.SoftDeleteMixin):
 
     hidden = sa.Column(sa.Boolean, default=False)
 
+    block_device_mapping = orm.relationship(
+        'BlockDeviceMapping',
+        back_populates='instance',
+        primaryjoin=(
+            'and_(BlockDeviceMapping.instance_uuid == Instance.uuid, '
+            'BlockDeviceMapping.deleted == 0)'
+        ),
+    )
+    console_auth_tokens = orm.relationship(
+        'ConsoleAuthToken',
+        back_populates='instance',
+        foreign_keys='ConsoleAuthToken.instance_uuid',
+        primaryjoin=(
+            'and_(Instance.uuid == ConsoleAuthToken.instance_uuid,'
+            'Instance.deleted == 0)'
+        ),
+    )
+    extra = orm.relationship(
+        'InstanceExtra',
+        back_populates='instance',
+        uselist=False,
+    )
+    info_cache = orm.relationship(
+        'InstanceInfoCache',
+        back_populates='instance',
+        uselist=False,
+    )
+    pci_devices = orm.relationship(
+        'PciDevice',
+        back_populates='instance',
+        foreign_keys='PciDevice.instance_uuid',
+        primaryjoin=(
+            'and_(Instance.uuid == PciDevice.instance_uuid,'
+            'PciDevice.deleted == 0)'
+        ),
+    )
+    services = orm.relationship(
+        'Service',
+        back_populates='instance',
+        primaryjoin=(
+            'and_(Instance.host == Service.host,'
+            'Service.binary == "nova-compute",'
+            'Instance.deleted == 0)'
+        ),
+        foreign_keys='Service.host',
+    )
+    security_groups = orm.relationship(
+        'SecurityGroup',
+        secondary='security_group_instance_association',
+        back_populates='instances',
+        primaryjoin=(
+            'and_('
+            'SecurityGroupInstanceAssociation.instance_uuid == Instance.uuid,'
+            # (anthony) the condition below shouldn't be necessary now that the
+            # association is being marked as deleted.  However, removing this
+            # may cause existing deployments to choke, so I'm leaving it
+            'Instance.deleted == 0)'
+        ),
+        secondaryjoin=(
+            'and_('
+            'SecurityGroup.id == SecurityGroupInstanceAssociation.security_group_id,'  # noqa: E501
+            'SecurityGroupInstanceAssociation.deleted == 0,'
+            'SecurityGroup.deleted == 0)'
+        ),
+    )
+    system_metadata = orm.relationship(
+        'InstanceSystemMetadata',
+        back_populates='instance',
+    )
+    tags = orm.relationship(
+        'Tag',
+        back_populates='instance',
+        primaryjoin=(
+            'and_(Instance.uuid == Tag.resource_id,Instance.deleted == 0)'
+        ),
+        foreign_keys='Tag.resource_id',
+    )
+
+
+# NOTE(stephenfin): https://github.com/sqlalchemy/sqlalchemy/discussions/8619
+Instance.metadata = orm.relationship(
+    'InstanceMetadata',
+    back_populates='instance',
+    foreign_keys='InstanceMetadata.instance_uuid',
+    primaryjoin=(
+        'and_(Instance.uuid == InstanceMetadata.instance_uuid,'
+        'InstanceMetadata.deleted == 0)'
+    ),
+)
+
 
 class InstanceInfoCache(BASE, NovaBase, models.SoftDeleteMixin):
     """Represents a cache of information about an instance
@@ -442,7 +532,7 @@ class InstanceInfoCache(BASE, NovaBase, models.SoftDeleteMixin):
     instance_uuid = sa.Column(sa.String(36), sa.ForeignKey('instances.uuid'),
                            nullable=False)
     instance = orm.relationship(Instance,
-                            backref=orm.backref('info_cache', uselist=False),
+                            back_populates='info_cache',
                             foreign_keys=instance_uuid,
                             primaryjoin=instance_uuid == Instance.uuid)
 
@@ -466,8 +556,7 @@ class InstanceExtra(BASE, NovaBase, models.SoftDeleteMixin):
     # and can be removed in the future release.
     resources = orm.deferred(sa.Column(sa.Text))
     instance = orm.relationship(Instance,
-                            backref=orm.backref('extra',
+                            back_populates='extra',
-                                                uselist=False),
                             foreign_keys=instance_uuid,
                             primaryjoin=instance_uuid == Instance.uuid)
 
@@ -616,7 +705,7 @@ class BlockDeviceMapping(BASE, NovaBase, models.SoftDeleteMixin):
     # transition period first.
     uuid = sa.Column(sa.String(36))
     instance = orm.relationship(Instance,
-                            backref=orm.backref('block_device_mapping'),
+                            back_populates='block_device_mapping',
                             foreign_keys=instance_uuid,
                             primaryjoin='and_(BlockDeviceMapping.'
                                               'instance_uuid=='
@@ -664,10 +753,9 @@ class BlockDeviceMapping(BASE, NovaBase, models.SoftDeleteMixin):
     encryption_format = sa.Column(sa.String(128))
     encryption_options = sa.Column(sa.String(4096))
 
+
 # TODO(stephenfin): Remove once we drop the security_groups field from the
 # Instance table. Until then, this is tied to the SecurityGroup table
-
-
 class SecurityGroupInstanceAssociation(BASE, NovaBase, models.SoftDeleteMixin):
     __tablename__ = 'security_group_instance_association'
     __table_args__ = (
@@ -698,6 +786,7 @@ class SecurityGroup(BASE, NovaBase, models.SoftDeleteMixin):
     project_id = sa.Column(sa.String(255))
 
     instances = orm.relationship(Instance,
+                             back_populates='security_groups',
                              secondary = "security_group_instance_association",
                              primaryjoin = 'and_('
         'SecurityGroup.id == '
@@ -709,8 +798,17 @@ class SecurityGroup(BASE, NovaBase, models.SoftDeleteMixin):
         # (anthony) the condition below shouldn't be necessary now that the
         # association is being marked as deleted.  However, removing this
         # may cause existing deployments to choke, so I'm leaving it
-        'Instance.deleted == 0)',
+        'Instance.deleted == 0)')
-                             backref='security_groups')
+
+    rules = orm.relationship(
+        'SecurityGroupIngressRule',
+        primaryjoin=(
+            'and_('
+            'SecurityGroupIngressRule.parent_group_id == SecurityGroup.id,'
+            'SecurityGroupIngressRule.deleted == 0)'
+        ),
+        back_populates='parent_group',
+    )
 
 
 # TODO(stephenfin): Remove once we drop the security_groups field from the
@@ -723,8 +821,9 @@ class SecurityGroupIngressRule(BASE, NovaBase, models.SoftDeleteMixin):
 
     parent_group_id = sa.Column(
         sa.Integer, sa.ForeignKey('security_groups.id'))
-    parent_group = orm.relationship("SecurityGroup", backref="rules",
+    parent_group = orm.relationship("SecurityGroup",
-                                foreign_keys=parent_group_id,
+                                back_populates="rules",
+                                foreign_keys=[parent_group_id],
                                 primaryjoin='and_('
         'SecurityGroupIngressRule.parent_group_id == SecurityGroup.id,'
         'SecurityGroupIngressRule.deleted == 0)')
@@ -738,7 +837,7 @@ class SecurityGroupIngressRule(BASE, NovaBase, models.SoftDeleteMixin):
     # granting access for.
     group_id = sa.Column(sa.Integer, sa.ForeignKey('security_groups.id'))
     grantee_group = orm.relationship("SecurityGroup",
-                                 foreign_keys=group_id,
+                                 foreign_keys=[group_id],
                                  primaryjoin='and_('
         'SecurityGroupIngressRule.group_id == SecurityGroup.id,'
         'SecurityGroupIngressRule.deleted == 0)')
@@ -824,12 +923,16 @@ class InstanceMetadata(BASE, NovaBase, models.SoftDeleteMixin):
     key = sa.Column(sa.String(255))
     value = sa.Column(sa.String(255))
     instance_uuid = sa.Column(sa.String(36), sa.ForeignKey('instances.uuid'))
-    instance = orm.relationship(Instance, backref="metadata",
+
-                            foreign_keys=instance_uuid,
+    instance = orm.relationship(
-                            primaryjoin='and_('
+        Instance,
-                                'InstanceMetadata.instance_uuid == '
+        back_populates="metadata",
-                                     'Instance.uuid,'
+        foreign_keys=instance_uuid,
-                                'InstanceMetadata.deleted == 0)')
+        primaryjoin=(
+            'and_(InstanceMetadata.instance_uuid == Instance.uuid,'
+            'InstanceMetadata.deleted == 0)'
+        ),
+    )
 
 
 class InstanceSystemMetadata(BASE, NovaBase, models.SoftDeleteMixin):
@@ -845,8 +948,11 @@ class InstanceSystemMetadata(BASE, NovaBase, models.SoftDeleteMixin):
                            sa.ForeignKey('instances.uuid'),
                            nullable=False)
 
-    instance = orm.relationship(Instance, backref="system_metadata",
+    instance = orm.relationship(
-                            foreign_keys=instance_uuid)
+        Instance,
+        back_populates='system_metadata',
+        foreign_keys=instance_uuid,
+    )
 
 
 class VolumeUsage(BASE, NovaBase, models.SoftDeleteMixin):
@@ -1022,7 +1128,8 @@ class PciDevice(BASE, NovaBase, models.SoftDeleteMixin):
     numa_node = sa.Column(sa.Integer, nullable=True)
 
     parent_addr = sa.Column(sa.String(12), nullable=True)
-    instance = orm.relationship(Instance, backref="pci_devices",
+    instance = orm.relationship(Instance,
+                                back_populates="pci_devices",
                                 foreign_keys=instance_uuid,
                                 primaryjoin='and_('
                                 'PciDevice.instance_uuid == Instance.uuid,'
@@ -1040,11 +1147,11 @@ class Tag(BASE, models.ModelBase):
     tag = sa.Column(sa.Unicode(80), primary_key=True, nullable=False)
 
     instance = orm.relationship(
-        "Instance",
+        'Instance',
-        backref='tags',
+        back_populates='tags',
+        foreign_keys=resource_id,
         primaryjoin='and_(Tag.resource_id == Instance.uuid,'
                     'Instance.deleted == 0)',
-        foreign_keys=resource_id
     )
 
 
@@ -1074,7 +1181,7 @@ class ConsoleAuthToken(BASE, NovaBase):
 
     instance = orm.relationship(
         "Instance",
-        backref='console_auth_tokens',
+        back_populates='console_auth_tokens',
         primaryjoin='and_(ConsoleAuthToken.instance_uuid == Instance.uuid,'
                     'Instance.deleted == 0)',
         foreign_keys=instance_uuid