Add ObjectGroup to the database model

This change adds a new SQLAlchemy object to the database model
representing the ObjectGroup attribute. The new object is linked
to the base ManagedObject object, allowing ObjectGroup attributes
to be associated with any managed object. A new unit test suite
has been added to verify the new object functionality and to
confirm that the object can be stored and retrieved from an
in-memory database.

kmip/pie/objects.py

@@ -48,6 +48,28 @@ app_specific_info_map = sqlalchemy.Table(
 )
 
 
+object_group_map = sqlalchemy.Table(
+    "object_group_map",
+    sql.Base.metadata,
+    sqlalchemy.Column(
+        "managed_object_id",
+        sqlalchemy.Integer,
+        sqlalchemy.ForeignKey(
+            "managed_objects.uid",
+            ondelete="CASCADE"
+        )
+    ),
+    sqlalchemy.Column(
+        "object_group_id",
+        sqlalchemy.Integer,
+        sqlalchemy.ForeignKey(
+            "object_groups.id",
+            ondelete="CASCADE"
+        )
+    )
+)
+
+
 class ManagedObject(sql.Base):
     """
     The abstract base class of the simplified KMIP object hierarchy.
@@ -92,6 +114,12 @@ class ManagedObject(sql.Base):
         back_populates="managed_objects",
         passive_deletes=True
     )
+    object_groups = sqlalchemy.orm.relationship(
+        "ObjectGroup",
+        secondary=object_group_map,
+        back_populates="managed_objects",
+        passive_deletes=True
+    )
 
     __mapper_args__ = {
         'polymorphic_identity': 'ManagedObject',
@@ -1841,3 +1869,64 @@ class ApplicationSpecificInformation(sql.Base):
             return not (self == other)
         else:
             return NotImplemented
+
+
+class ObjectGroup(sql.Base):
+    __tablename__ = "object_groups"
+    id = sqlalchemy.Column(sqlalchemy.Integer, primary_key=True)
+    _object_group = sqlalchemy.Column(
+        "object_group",
+        sqlalchemy.String,
+        nullable=False,
+        unique=True
+    )
+    managed_objects = sqlalchemy.orm.relationship(
+        "ManagedObject",
+        secondary=object_group_map,
+        back_populates="object_groups"
+    )
+
+    def __init__(self, object_group=None):
+        """
+        Create an ObjectGroup attribute.
+
+        Args:
+            object_group (str): A string specifying the object group. Required.
+        """
+        super(ObjectGroup, self).__init__()
+
+        self.object_group = object_group
+
+    @property
+    def object_group(self):
+        return self._object_group
+
+    @object_group.setter
+    def object_group(self, value):
+        if (value is None) or (isinstance(value, six.string_types)):
+            self._object_group = value
+        else:
+            raise TypeError("The object group must be a string.")
+
+    def __repr__(self):
+        object_group = "object_group={}".format(self.object_group)
+
+        return "ObjectGroup({})".format(object_group)
+
+    def __str__(self):
+        return str({"object_group": self.object_group})
+
+    def __eq__(self, other):
+        if isinstance(other, ObjectGroup):
+            if self.object_group != other.object_group:
+                return False
+            else:
+                return True
+        else:
+            return NotImplemented
+
+    def __ne__(self, other):
+        if isinstance(other, ObjectGroup):
+            return not (self == other)
+        else:
+            return NotImplemented