Fix bug on UserGroupAssoc mapping (see note)

This is the same bug as the one fixed on the main model in
https://github.com/galaxyproject/galaxy/commit/174d8b53ac4d48ac6950c579539537da42a6a2e8

lib/tool_shed/grids/admin_grids.py

@@ -259,8 +259,8 @@ class GroupGrid(grids.Grid):
     class UsersColumn(grids.GridColumn):
 
         def get_value(self, trans, grid, group):
-            if group.members:
-                return len(group.members)
+            if group.users:
+                return len(group.users)
             return 0
 
     title = "Groups"

lib/tool_shed/webapp/model/__init__.py

@@ -174,7 +174,7 @@ class Group(Base, Dictifiable, _HasTable):
     name = Column(String(255), index=True, unique=True)
     deleted = Column(Boolean, index=True, default=False)
     roles = relationship('GroupRoleAssociation', back_populates='group')
-    users = relationship('UserGroupAssociation')  # TODO: fix bug: incorrect usage of backref/duplicate rel users/members; add back_populates
+    users = relationship('UserGroupAssociation', back_populates='group')
 
     dict_collection_visible_keys = ['id', 'name']
     dict_element_visible_keys = ['id', 'name']

lib/tool_shed/webapp/model/mapping.py

@@ -108,7 +108,7 @@ mapper_registry.map_imperatively(RepositoryRoleAssociation, RepositoryRoleAssoci
 
 mapper_registry.map_imperatively(UserGroupAssociation, UserGroupAssociation.table,
        properties=dict(user=relation(User, backref="groups"),
-           group=relation(Group, backref="members")))  # TODO fix bug: members should be users; check codebase for references
+           group=relation(Group, back_populates="users")))
 
 mapper_registry.map_imperatively(UserRoleAssociation, UserRoleAssociation.table,
        properties=dict(

lib/tool_shed/webapp/security/__init__.py

@@ -234,7 +234,7 @@ class CommunityRBACAgent(RBACAgent):
                         # of a group that is associated with the role.
                         for gra in role.groups:
                             group = gra.group
-                            for uga in group.members:
+                            for uga in group.users:
                                 member = uga.user
                                 if member.id == user.id:
                                     return True