Use `TextClause` to define the DB model "server_default"

The alembic comparator needs the same class to compare the
database model with the loaded SQL schema.

Closes-Bug: #2020050
Change-Id: I4feab3e55f2e38a9eca70d4ca755b4c2974346d5

neutron/common/_constants.py

@@ -14,6 +14,7 @@
 # limitations under the License.
 
 from neutron_lib import constants
+from sqlalchemy.sql import elements
 
 # NOTE(boden): This module is common constants for neutron only.
 # Any constants used outside of neutron should go into neutron-lib.
@@ -92,3 +93,6 @@ METADATA_V6_CIDR = constants.METADATA_V6_IP + '/128'
 
 # TODO(haleyb): move this constant to neutron_lib.constants
 IPV4_MIN_MTU = 68
+
+# TODO(ralonsoh): move to neutron-lib.db.contants
+SQL_EMPTY_STRING = elements.TextClause("''")

neutron/db/models/dns.py

@@ -16,6 +16,7 @@ import sqlalchemy as sa
 from sqlalchemy import orm
 from sqlalchemy import sql
 
+from neutron.common import _constants as n_const
 from neutron.db.models import l3 as l3_models
 from neutron.db import models_v2
 
@@ -88,7 +89,7 @@ class PortDNS(model_base.BASEV2):
     dns_name = sa.Column(sa.String(255), nullable=False)
     dns_domain = sa.Column(sa.String(constants.FQDN_FIELD_SIZE),
                            nullable=False,
-                           server_default='')
+                           server_default=n_const.SQL_EMPTY_STRING)
     # Add a relationship to the Port model in order to instruct
     # SQLAlchemy to eagerly load this association
     port = orm.relationship(models_v2.Port,

neutron/db/models_v2.py

@@ -25,6 +25,7 @@ import sqlalchemy as sa
 from sqlalchemy import orm
 from sqlalchemy import sql
 
+from neutron.common import _constants as n_const
 from neutron.db.network_dhcp_agent_binding import models as ndab_model
 from neutron.db import rbac_db_models
 
@@ -296,7 +297,8 @@ class SubnetPool(standard_attr.HasStandardAttributes, model_base.BASEV2,
     is_default = sa.Column(sa.Boolean, nullable=False,
                            server_default=sql.false())
     default_quota = sa.Column(sa.Integer, nullable=True)
-    hash = sa.Column(sa.String(36), nullable=False, server_default='')
+    hash = sa.Column(sa.String(36), nullable=False,
+                     server_default=n_const.SQL_EMPTY_STRING)
     address_scope_id = sa.Column(sa.String(36), nullable=True, index=True)
     prefixes = orm.relationship(SubnetPoolPrefix,
                                 backref='subnetpools',

neutron/plugins/ml2/models.py

@@ -19,6 +19,7 @@ from neutron_lib.db import model_base
 import sqlalchemy as sa
 from sqlalchemy import orm
 
+from neutron.common import _constants as n_const
 from neutron.db.models import segment as segment_models
 from neutron.db import models_v2
 
@@ -45,10 +46,10 @@ class PortBinding(model_base.BASEV2):
                           default=portbindings.VNIC_NORMAL,
                           server_default=portbindings.VNIC_NORMAL)
     profile = sa.Column(sa.String(BINDING_PROFILE_LEN), nullable=False,
-                        default='', server_default='')
+                        default='', server_default=n_const.SQL_EMPTY_STRING)
     vif_type = sa.Column(sa.String(64), nullable=False)
     vif_details = sa.Column(sa.String(4095), nullable=False, default='',
-                            server_default='')
+                            server_default=n_const.SQL_EMPTY_STRING)
     status = sa.Column(sa.String(16), nullable=False,
                        default=constants.ACTIVE,
                        server_default=constants.ACTIVE)
@@ -115,12 +116,12 @@ class DistributedPortBinding(model_base.BASEV2):
     router_id = sa.Column(sa.String(36), nullable=True)
     vif_type = sa.Column(sa.String(64), nullable=False)
     vif_details = sa.Column(sa.String(4095), nullable=False, default='',
-                            server_default='')
+                            server_default=n_const.SQL_EMPTY_STRING)
     vnic_type = sa.Column(sa.String(64), nullable=False,
                           default=portbindings.VNIC_NORMAL,
                           server_default=portbindings.VNIC_NORMAL)
     profile = sa.Column(sa.String(BINDING_PROFILE_LEN), nullable=False,
-                        default='', server_default='')
+                        default='', server_default=n_const.SQL_EMPTY_STRING)
     status = sa.Column(sa.String(16), nullable=False)
 
     # Add a relationship to the Port model in order to instruct SQLAlchemy to