diff --git a/app/models/profile.rb b/app/models/profile.rb
index 3da3062..fac0e08 100644
--- a/app/models/profile.rb
+++ b/app/models/profile.rb
@@ -3,6 +3,9 @@
 # which by default is the one returned by VirtualCommunity:default.
 class Profile < ActiveRecord::Base
 
+  # Valid identifiers must match this format.
+  IDENTIFIER_FORMAT = /^[a-z][a-z0-9_]+[a-z0-9]$/
+
   # These names cannot be used as identifiers for Profiles
   RESERVED_IDENTIFIERS = %w[
   admin
@@ -17,10 +20,26 @@ class Profile < ActiveRecord::Base
   belongs_to :virtual_community
   belongs_to :user
 
+
+  # Sets the identifier for this profile. Raises an exception when called on a
+  # existing profile (since profiles cannot be renamed)
+  def identifier=(value)
+    unless self.new_record?
+      raise ArgumentError.new(_('An existing profile cannot be renamed.'))
+    end
+    self[:identifier] = value
+  end
+
   validates_presence_of :identifier
-  validates_format_of :identifier, :with => /^[a-z][a-z0-9_]+[a-z0-9]$/
+  validates_format_of :identifier, :with => IDENTIFIER_FORMAT
   validates_exclusion_of :identifier, :in => RESERVED_IDENTIFIERS
 
+  # A user cannot have more than one profile, but many profiles can exist
+  # without being associated to a particular user.
+  validates_uniqueness_of :user_id, :if => (lambda do |profile|
+    ! profile.user_id.nil?
+  end)
+
   # creates a new Profile. By default, it is attached to the default
   # VirtualCommunity (see VirtualCommunity#default), unless you tell it
   # otherwise
diff --git a/app/models/user.rb b/app/models/user.rb
index dce0d43..77b4156 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -1,4 +1,5 @@
 require 'digest/sha1'
+
 class User < ActiveRecord::Base
 
   after_create do |user|
@@ -10,6 +11,7 @@ class User < ActiveRecord::Base
   attr_accessor :password
 
   validates_presence_of     :login, :email
+  validates_format_of       :login, :with => Profile::IDENTIFIER_FORMAT
   validates_presence_of     :password,                   :if => :password_required?
   validates_presence_of     :password_confirmation,      :if => :password_required?
   validates_length_of       :password, :within => 4..40, :if => :password_required?
diff --git a/test/unit/profile_test.rb b/test/unit/profile_test.rb
index d990ea9..10f15e7 100644
--- a/test/unit/profile_test.rb
+++ b/test/unit/profile_test.rb
@@ -50,4 +50,31 @@ class ProfileTest < Test::Unit::TestCase
     assert p.valid?
   end
 
+  def test_only_one_profile_per_user
+    p1 = profiles(:johndoe)
+    assert_equal users(:johndoe), p1.user
+    
+    p2 = Profile.new
+    p2.user = users(:johndoe)
+    assert !p2.valid?
+    assert p2.errors.invalid?(:user_id)
+  end
+
+  def test_several_profiles_without_user
+    p1 = profiles(:john_and_joe)
+    assert p1.valid?
+    assert_nil p1.user
+
+    p2 = Profile.new
+    assert !p2.valid?
+    assert !p2.errors.invalid?(:user_id)
+  end
+
+  def test_cannot_rename
+    p1 = profiles(:johndoe)
+    assert_raise ArgumentError do
+      p1.identifier = 'bli'
+    end
+  end
+
 end
diff --git a/test/unit/user_test.rb b/test/unit/user_test.rb
index be238bc..12280a4 100644
--- a/test/unit/user_test.rb
+++ b/test/unit/user_test.rb
@@ -75,10 +75,37 @@ class UserTest < Test::Unit::TestCase
     user = User.create!(:login => 'new_user', :email => 'new_user@example.com', :password => 'test', :password_confirmation => 'test')
 
     assert Profile.exists?(['user_id = ?', user.id])
+
     assert_equal users_count + 1, User.count
     assert_equal profiles_count + 1, Profile.count
   end
 
+  def test_login_validation
+    u = User.new
+    u.valid?
+    assert u.errors.invalid?(:login)
+
+    u.login = 'with space'
+    u.valid?
+    assert u.errors.invalid?(:login)
+
+    u.login = 'áéíóú'
+    u.valid?
+    assert u.errors.invalid?(:login)
+
+    u.login = 'rightformat2007'
+    u.valid?
+    assert ! u.errors.invalid?(:login)
+
+    u.login = 'rightformat'
+    u.valid?
+    assert ! u.errors.invalid?(:login)
+
+    u.login = 'right_format'
+    u.valid?
+    assert ! u.errors.invalid?(:login)
+  end
+
   protected
     def create_user(options = {})
       User.create({ :login => 'quire', :email => 'quire@example.com', :password => 'quire', :password_confirmation => 'quire' }.merge(options))
--
libgit2 0.21.2