I'm sorry, but I'm new to Mongodb. I can not understand how to design the
next model.
Collection of users:
[
    {name:'user1', password:'123',
groups:['all_users','author','forum_admin']},
    {name:'user2', password:'123', groups:['all_users','author',]},
    {name:'user3', password:'123', groups:['anonim']}
]
Hierarchy of user groups:
[
    {  _id:'all_users',
       name:'All users',
       groups:[
           {_id:'author', name:'Author'}
           {_id:'anonim', name:'Anonimous user'}
           { _id:'admin',
             name:'Administrator',
             groups:[
                 {_id:'sa', name:'Super administrator'},
                 {_id:'sys_admin', name:'System administrator'},
                 {_id:'forum_admin', name:'Forum administrator'}
             ]
           }
       ]
    }
]

How can I associate users and groups? I tried to add in each user a list of
groups. But I have a problem. *H**ow to extract the group information from
the group hierarchy by them _id?* Request
db.groups.find({_id:'forum_admin'}) return empty list.

Hi Andrey,

Just to clarify: Does your groups collection only consist of a single
document where the _id is 'all_users' ? Are you embedding the hierarchy of
all groups into that single document? It looks like that from what you
stated as your Hierarchy of user groups. That would also explain why you
get an empty result when you query for a group with the _id of
"forum_admin", because there is no such document that has that _id on the
top level.

There are several ways to store a group hierarchy in MongoDB:

   - You can use that one-document approach, but then you need to pull the
   complete document to the client side and search the tree for a certain
   subgroup within your application.
   - You can insert one document for each group into the groups collection.
   If you want to keep the hierarchy information, you can add a "parent" field
   that points to the parent group.
   - As above, but instead of pointing to the parent, you can keep the tree
   of sub-groups as a "children" sub-document.

It really depends on what you're planning to do exactly and how your use
case is for the groups. Think about the different situations when you need
access to the groups collection and what answers you want to find. For
example, do you really need the hierarchy every time you query the groups
collection? Or do you mostly just want to get some extra information or
find certain rights of a user's group? There is no one single right answer,
it really depends what you're trying to achieve.

Regards,
Thomas

Thank you for your answer. I decided to use the second method (which is
very similar to the implementation of a relational database). I just thought
that Mongo can do it somehow more elegant.