Add host label PUT support

host-label-put.diff

diff --git a/craton/api/v1/resources/inventory/hosts.py b/craton/api/v1/resources/inventory/hosts.py
index 39264a5..39be56a 100644
--- a/craton/api/v1/resources/inventory/hosts.py
+++ b/craton/api/v1/resources/inventory/hosts.py
@@ -97,10 +97,10 @@ class HostsLabels(base.Resource):
         Update existing device label entirely, or add if it does
         not exist.
         """
-        print(request.json)
+        import sys; print(request.json, file=sys.stderr)
         context = request.environ.get('context')
-        dbapi.hosts_labels_update(context, id, request.json)
-        return request.json, 200, None
+        labels_obj = dbapi.hosts_labels_update(context, id, request.json)
+        return jsonutils.to_primitive(labels_obj), 200, None
 
     @base.http_codes
     def delete(self, id):
diff --git a/craton/db/sqlalchemy/alembic/versions/ffdc1a500db1_craton_inventory_init.py b/craton/db/sqlalchemy/alembic/versions/ffdc1a500db1_craton_inventory_init.py
index 1308830..74350b4 100644
--- a/craton/db/sqlalchemy/alembic/versions/ffdc1a500db1_craton_inventory_init.py
+++ b/craton/db/sqlalchemy/alembic/versions/ffdc1a500db1_craton_inventory_init.py
@@ -55,6 +55,7 @@ def upgrade():
         sa.Column('variable_association_id', sa.Integer),
         sa.Column('label', sa.String(length=255), nullable=True),
         sa.PrimaryKeyConstraint('id'),
+        sa.UniqueConstraint('label'),
         sa.ForeignKeyConstraint(
             ['variable_association_id'], ['variable_association.id'],
             'fk_labels_variable_association')
diff --git a/craton/db/sqlalchemy/api.py b/craton/db/sqlalchemy/api.py
index 3827c97..aefdf4e 100644
--- a/craton/db/sqlalchemy/api.py
+++ b/craton/db/sqlalchemy/api.py
@@ -9,6 +9,7 @@ from oslo_db.sqlalchemy import utils as db_utils
 from oslo_log import log
 
 import sqlalchemy.orm.exc as sa_exc
+from sqlalchemy.exc import IntegrityError
 from sqlalchemy.orm import with_polymorphic
 
 from craton import exceptions
@@ -457,6 +458,7 @@ def hosts_data_delete(context, host_id, data):
 def hosts_labels_update(context, host_id, labels):
     """Update labels for host. Add the label if it is not present
     in host labels list, otherwise do nothing."""
+    print('host_id', host_id)
     session = get_session()
     with session.begin():
         host_devices = with_polymorphic(models.Device, '*')
@@ -466,14 +468,32 @@ def hosts_labels_update(context, host_id, labels):
         query = query.filter_by(id=host_id)
         try:
             host = query.one()
-        except sa_exc.NoResultFound:
-            raise exceptions.NotFound()
-
-        present_labels = [l.label for l in host.labels]
-        to_update = [i for i in labels["labels"] if i not in present_labels]
-        _labels = [models.Label(i) for i in to_update]
-        host.labels.update(_labels)
+            print('host', host)
+        except sa_exc.NoResultFound as e:
+            raise exceptions.NotFound() from e
+
+        labels = set(labels)
+        add_to_host_labels = labels - set(host.associated_labels)
+
+        if not add_to_host_labels:
+            return set()
+
+        # Get existing labels from the database in the context of this
+        # transaction. NOTE: it is possible that another session could
+        # be updating simultaneously, in which case this query -- and
+        # corresponding REST API call -- would have to be re-applied
+        # again.
+        existing_label_objs = session.query(models.Label).filter(
+            models.Label.label.in_(add_to_host_labels)).all()
+        new_label_obs = set()
+        existing_labels = {label.label for label in existing_label_objs}
+        for label in add_to_host_labels:
+            if label not in existing_labels:
+                label_obj = models.Label(label)
+                host.labels.add(label_obj)
+        host.labels.update(existing_label_objs)
         host.save(session)
+        return add_to_host_labels
 
 
 def hosts_labels_delete(context, host_id, labels):
diff --git a/craton/db/sqlalchemy/models.py b/craton/db/sqlalchemy/models.py
index 920663e..ee0d2a8 100644
--- a/craton/db/sqlalchemy/models.py
+++ b/craton/db/sqlalchemy/models.py
@@ -425,7 +425,7 @@ class Label(Base, VariableMixin):
     """
     __tablename__ = 'labels'
     id = Column(Integer, primary_key=True)
-    label = Column(String(255), unique=False)
+    label = Column(String(255), unique=True)
 
     _repr_columns = [label]