action groups index.cr

class Groups::Index < BrowserAction
  route do
    groups = GroupQuery.new.join_users

    render Groups::IndexPage, groups: groups
  end
end

models group.cr

class Group < BaseModel
  table :groups do
    column title : String
    has_many users : User, through: :memberships
  end
end

models membership.cr

class Memebership < BaseModel
  table :memeberships do
    belongs_to group : Group
    belongs_to user : User
  end
end

models user.cr

class User < BaseModel
  table :users do
    # other columns
    has_many groups : Group, through: :memberships
  end
end