diff --git a/app/models/principal.rb b/app/models/principal.rb
index 829169ba76..bfe3ec4947 100644
--- a/app/models/principal.rb
+++ b/app/models/principal.rb
@@ -77,7 +77,7 @@ class Principal < ApplicationRecord
   scope :not_in_project, ->(project) {
     where.not(id: Member.of(project).select(:user_id))
   }
-  
+
   scope :in_group, ->(group) {
     within_group(group)
   }
diff --git a/app/models/queries/principals/filters/member_filter.rb b/app/models/queries/principals/filters/member_filter.rb
index f4303f76bc..1ccb596aa8 100644
--- a/app/models/queries/principals/filters/member_filter.rb
+++ b/app/models/queries/principals/filters/member_filter.rb
@@ -44,18 +44,21 @@ class Queries::Principals::Filters::MemberFilter < Queries::Principals::Filters:
   end
 
   def scope
-    default_scope = Principal.includes(:members)
-
     case operator
     when '='
-      default_scope.where(members: { project_id: values })
+      Principal.in_project(values)
     when '!'
-      default_scope.where.not(members: { project_id: values })
-        .or(default_scope.where(members: { project_id: nil }))
+      Principal.not_in_project(values)
     when '*'
-      default_scope.where.not(members: { project_id: nil })
+      member_included_scope.where.not(members: { id: nil })
     when '!*'
-      default_scope.where.not(id: Member.select(:user_id).uniq)
+      member_included_scope.where.not(id: Member.distinct(:user_id).select(:user_id))
     end
   end
+
+  private
+
+  def member_included_scope
+    Principal.includes(:members)
+  end
 end
diff --git a/docs/development/create-openproject-plugin/README.md b/docs/development/create-openproject-plugin/README.md
index 94b4c3ba04..ce3b736636 100644
--- a/docs/development/create-openproject-plugin/README.md
+++ b/docs/development/create-openproject-plugin/README.md
@@ -38,6 +38,10 @@ Once you've done that install it via
 bundle install
 ```
 
+### Production
+
+To use your plugin in production you have to add it to your [docker](../../installation-and-operations/installation/docker/#openproject-plugins) container or install it in the [packaged installation](../../installation-and-operations/configuration/plugins/#adding-plugins-debrpm-packages).
+
 ## Start coding
 
 You may have a look at some existing OpenProject plugins to get inspiration. It is possible to add new routes, views, models, … and/or overwrite existing ones.
diff --git a/spec/models/queries/principals/principal_query_integrations_spec.rb b/spec/models/queries/principals/principal_query_integrations_spec.rb
new file mode 100644
index 0000000000..de71305203
--- /dev/null
+++ b/spec/models/queries/principals/principal_query_integrations_spec.rb
@@ -0,0 +1,88 @@
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) 2012-2021 the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See docs/COPYRIGHT.rdoc for more details.
+#++
+
+require 'spec_helper'
+
+describe Queries::Principals::PrincipalQuery, 'integration', type: :model do
+  let(:current_user) { FactoryBot.create(:user) }
+  let(:instance) { described_class.new }
+
+  before do
+    login_as(current_user)
+  end
+
+  context 'with a member filter' do
+    let(:project) { FactoryBot.create(:project) }
+    let(:role) { FactoryBot.create(:role) }
+    let(:project_user) do
+      FactoryBot.create(:user,
+                        member_in_project: project,
+                        member_through_role: role) do |u|
+        # Granting another membership in order to better test the "not" filter
+        FactoryBot.create(:member,
+                          principal: u,
+                          project: other_project,
+                          roles: [role])
+      end
+    end
+    let(:other_project) { FactoryBot.create(:project) }
+    let(:other_project_user) do
+      FactoryBot.create(:user,
+                        member_in_project: other_project,
+                        member_through_role: role)
+    end
+
+    let(:users) { [current_user, project_user, other_project_user] }
+
+    before do
+      users
+    end
+
+    context 'with the = operator' do
+      before do
+        instance.where('member', '=', [project.id.to_s])
+      end
+
+      it 'returns all principals being member' do
+        expect(instance.results)
+          .to match_array [project_user]
+      end
+    end
+
+    context 'with the ! operator' do
+      before do
+        instance.where('member', '!', [project.id.to_s])
+      end
+
+      it 'returns all principals not being member' do
+        expect(instance.results)
+          .to match_array [current_user, other_project_user]
+      end
+    end
+  end
+end