FEATURE: `group_removes_trust_level` setting

By default in Discourse, if a group grants a user a particular trust
level that is locked even if they are removed from the group.

With this new setting, when a user is removed from a group their
trust level is set to either the next highest trust level based on group
membership, or they are unlocked and promoted based on the default
mechanisms.

app/models/group_user.rb

@@ -8,7 +8,7 @@ class GroupUser < ActiveRecord::Base
   after_destroy :remove_title
 
   after_save :set_primary_group
-  after_destroy :remove_primary_group
+  after_destroy :remove_primary_group, :recalculate_trust_level
 
   before_create :set_notification_level
   after_save :grant_trust_level
@@ -67,6 +67,30 @@ class GroupUser < ActiveRecord::Base
     return if group.grant_trust_level.nil?
     TrustLevelGranter.grant(group.grant_trust_level, user)
   end
+
+  def recalculate_trust_level
+    return if group.grant_trust_level.nil?
+    return unless SiteSetting.group_removes_trust_level?
+
+    # Find the highest level of the user's remaining groups
+    highest_level = GroupUser
+      .where(user_id: user.id)
+      .includes(:group)
+      .maximum("groups.grant_trust_level")
+
+    if highest_level.nil?
+      # If the user no longer has a group with a trust level,
+      # unlock them, start at 0 and consider promotions.
+      user.trust_level_locked = false
+      user.trust_level = 0
+      user.save
+
+      Promotion.new(user).review
+    else
+      user.change_trust_level!(highest_level)
+    end
+  end
+
 end
 
 # == Schema Information

config/site_settings.yml

@@ -908,6 +908,8 @@ trust:
   tl3_links_no_follow:
     default: false
     client: true
+  group_removes_trust_level:
+    default: false
 
 security:
   force_https:

spec/models/group_spec.rb

@@ -468,6 +468,67 @@ describe Group do
     expect(u3.reload.trust_level).to eq(3)
   end
 
+  describe "group_removes_trust_level" do
+
+    context "set to false" do
+      before do
+        SiteSetting.group_removes_trust_level = false
+      end
+
+      it "maintains the chosen trust level" do
+        g0 = Fabricate(:group, grant_trust_level: 2)
+        g1 = Fabricate(:group, grant_trust_level: 3)
+        user = Fabricate(:user, trust_level: 0)
+
+        g0.add(user)
+        expect(user.reload.trust_level).to eq(2)
+
+        g1.add(user)
+        expect(user.reload.trust_level).to eq(3)
+
+        g1.remove(user)
+        expect(user.reload.trust_level).to eq(3)
+
+        g0.remove(user)
+        expect(user.reload.trust_level).to eq(3)
+      end
+    end
+
+    context "set to true" do
+      before do
+        SiteSetting.group_removes_trust_level = true
+      end
+
+      it "adjusts the user trust level" do
+        g0 = Fabricate(:group, grant_trust_level: 2)
+        g1 = Fabricate(:group, grant_trust_level: 3)
+        g2 = Fabricate(:group)
+
+        # Add a group without one to consider `NULL` check
+        g2.add(user)
+
+        user = Fabricate(:user, trust_level: 0)
+
+        g0.add(user)
+        expect(user.reload.trust_level).to eq(2)
+        expect(user.trust_level_locked?).to eq(true)
+
+        g1.add(user)
+        expect(user.reload.trust_level).to eq(3)
+        expect(user.trust_level_locked?).to eq(true)
+
+        g1.remove(user)
+        expect(user.reload.trust_level).to eq(2)
+        expect(user.trust_level_locked?).to eq(true)
+
+        g0.remove(user)
+        expect(user.reload.trust_level).to eq(0)
+        expect(user.trust_level_locked?).to eq(false)
+      end
+    end
+
+  end
+
   it 'should cook the bio' do
     group = Fabricate(:group)
     group.update_attributes!(bio_raw: 'This is a group for :unicorn: lovers')