NoSQL & Firebase: Storing & Iterating over many-to-many relationship

Question

I have 2 main nodes in my database: Users and Projects.

Each user can be assigned to multiple projects, in different roles.

After reading about how to structure many to many relationship, I ended up that it should look like this:

users : {
  user1 : {
    name : blah,
    email : a@a.com,
    projects : {
      projects1key : true,
      projects2key : true
    }
  }
}

projects : {
  project1key : {
    name : blahserve,
    category : blahbers,
    providers : {
      user1 : true,
      user7 : true
    }
  }
}

What I couldn't figure out is how I can assign every user a role in each project.

1. What's the new database structure should look like if I need to add role (string) to each user within a project?

2. Correct me if I'm wrong: When I assign user to a project, I need to create 2 new nodes: a projectkey node in my user node, and a userkey node in my projectkeynode. Is that right?

Update

Since the answer given here is correct but not sure it will fit my use case, this is my use case:

• (1) Iterating over a list of all project users
• (2) Iterating over a list of all member project
• (3) Check for project role when the user access a project and give give him permission depends on his role
• (4) The "Project -> users" page allow you to add a new user to existing project. the user role is picked in the same form together with the user. the user must have a role in a project.

Answer 1

Or maybe just simply you could add a different node all together:-

What's the new database structure should look like if I need to add role (string) to each user within a project?

users : {
 user1 : {
  name : blah,
  email : a@a.com,
    }
  },

projects : {
  projectKey1 : {
    name : blahserve,
    category : blahbers,
    }
},

projectsLists:{

  user1 :{
    projectKey1 : true,
    projectKey2 : true 
    }
}, 

projectsRoles :{

  projectKey1 : {
     user1 : provider,
     user2 : editor,
     ....
    }
  }

Correct me if I'm wrong: When I assign user to a project, I need to create 2 new nodes: a projectkey node in my user node, and a userkey node in my projectkeynode. Is that right?

Always prefer the flatter DB structure. So using this structure you only gotta append or update your user's role once.