diff --git a/app/controllers/application_controller.rb b/app/controllers/application_controller.rb
index 7c8c1392c1c..6f0dc2a3a20 100644
--- a/app/controllers/application_controller.rb
+++ b/app/controllers/application_controller.rb
@@ -12,9 +12,6 @@ class ApplicationController < ActionController::Base
   include EnforcesTwoFactorAuthentication
   include WithPerformanceBar
   include SessionlessAuthentication
-  # this can be removed after switching to rails 5
-  # https://gitlab.com/gitlab-org/gitlab-ce/issues/51908
-  include InvalidUTF8ErrorHandler unless Gitlab.rails5?
 
   before_action :authenticate_user!
   before_action :enforce_terms!, if: :should_enforce_terms?
@@ -157,7 +154,7 @@ class ApplicationController < ActionController::Base
   def log_exception(exception)
     Gitlab::Sentry.track_acceptable_exception(exception)
 
-    backtrace_cleaner = Gitlab.rails5? ? request.env["action_dispatch.backtrace_cleaner"] : env
+    backtrace_cleaner = request.env["action_dispatch.backtrace_cleaner"]
     application_trace = ActionDispatch::ExceptionWrapper.new(backtrace_cleaner, exception).application_trace
     application_trace.map! { |t| "  #{t}\n" }
     logger.error "\n#{exception.class.name} (#{exception.message}):\n#{application_trace.join}"
diff --git a/app/controllers/concerns/invalid_utf8_error_handler.rb b/app/controllers/concerns/invalid_utf8_error_handler.rb
deleted file mode 100644
index 44c6d6b0da0..00000000000
--- a/app/controllers/concerns/invalid_utf8_error_handler.rb
+++ /dev/null
@@ -1,27 +0,0 @@
-# frozen_string_literal: true
-
-module InvalidUTF8ErrorHandler
-  extend ActiveSupport::Concern
-
-  included do
-    rescue_from ArgumentError, with: :handle_invalid_utf8
-  end
-
-  private
-
-  def handle_invalid_utf8(error)
-    if error.message == "invalid byte sequence in UTF-8"
-      render_412
-    else
-      raise(error)
-    end
-  end
-
-  def render_412
-    respond_to do |format|
-      format.html { render "errors/precondition_failed", layout: "errors", status: 412 }
-      format.js { render json: { error: 'Invalid UTF-8' }, status: :precondition_failed, content_type: 'application/json' }
-      format.any { head :precondition_failed }
-    end
-  end
-end
diff --git a/app/models/ci/pipeline.rb b/app/models/ci/pipeline.rb
index 2cdb4780412..25937065011 100644
--- a/app/models/ci/pipeline.rb
+++ b/app/models/ci/pipeline.rb
@@ -56,11 +56,7 @@ module Ci
     validates :tag, inclusion: { in: [false], if: :merge_request? }
     validates :status, presence: { unless: :importing? }
     validate :valid_commit_sha, unless: :importing?
-
-    # Replace validator below with
-    # `validates :source, presence: { unless: :importing? }, on: :create`
-    # when removing Gitlab.rails5? code.
-    validate :valid_source, unless: :importing?, on: :create
+    validates :source, exclusion: { in: %w(unknown), unless: :importing? }, on: :create
 
     after_create :keep_around_commits, unless: :importing?
 
@@ -738,11 +734,5 @@ module Ci
 
       project.repository.keep_around(self.sha, self.before_sha)
     end
-
-    def valid_source
-      if source.nil? || source == "unknown"
-        errors.add(:source, "invalid source")
-      end
-    end
   end
 end
diff --git a/app/models/ci/runner.rb b/app/models/ci/runner.rb
index 2693386443a..3e5cedf92b9 100644
--- a/app/models/ci/runner.rb
+++ b/app/models/ci/runner.rb
@@ -58,8 +58,7 @@ module Ci
 
     # BACKWARD COMPATIBILITY: There are needed to maintain compatibility with `AVAILABLE_SCOPES` used by `lib/api/runners.rb`
     scope :deprecated_shared, -> { instance_type }
-    # this should get replaced with `project_type.or(group_type)` once using Rails5
-    scope :deprecated_specific, -> { where(runner_type: [runner_types[:project_type], runner_types[:group_type]]) }
+    scope :deprecated_specific, -> { project_type.or(group_type) }
 
     scope :belonging_to_project, -> (project_id) {
       joins(:runner_projects).where(ci_runner_projects: { project_id: project_id })
diff --git a/app/models/concerns/enum_with_nil.rb b/app/models/concerns/enum_with_nil.rb
index 23acfe9a55f..6d0a21cf070 100644
--- a/app/models/concerns/enum_with_nil.rb
+++ b/app/models/concerns/enum_with_nil.rb
@@ -16,7 +16,7 @@ module EnumWithNil
         # E.g. for enum_with_nil failure_reason: { unknown_failure: nil }
         # this overrides auto-generated method `unknown_failure?`
         define_method("#{key_with_nil}?") do
-          Gitlab.rails5? ? self[name].nil? : super()
+          self[name].nil?
         end
 
         # E.g. for enum_with_nil failure_reason: { unknown_failure: nil }
@@ -24,7 +24,6 @@ module EnumWithNil
         define_method(name) do
           orig = super()
 
-          return orig unless Gitlab.rails5?
           return orig unless orig.nil?
 
           self.class.public_send(name.to_s.pluralize).key(nil) # rubocop:disable GitlabSecurity/PublicSend
diff --git a/app/models/concerns/redis_cacheable.rb b/app/models/concerns/redis_cacheable.rb
index 69554f18ea2..4bb4ffe2a8e 100644
--- a/app/models/concerns/redis_cacheable.rb
+++ b/app/models/concerns/redis_cacheable.rb
@@ -49,10 +49,6 @@ module RedisCacheable
   end
 
   def cast_value_from_cache(attribute, value)
-    if Gitlab.rails5?
-      self.class.type_for_attribute(attribute.to_s).cast(value)
-    else
-      self.class.column_for_attribute(attribute).type_cast_from_database(value)
-    end
+    self.class.type_for_attribute(attribute.to_s).cast(value)
   end
 end
diff --git a/app/models/event.rb b/app/models/event.rb
index 2ceef412af5..6a35bca72c5 100644
--- a/app/models/event.rb
+++ b/app/models/event.rb
@@ -114,19 +114,6 @@ class Event < ActiveRecord::Base
       end
     end
 
-    # Remove this method when removing Gitlab.rails5? code.
-    def subclass_from_attributes(attrs)
-      return super if Gitlab.rails5?
-
-      # Without this Rails will keep calling this method on the returned class,
-      # resulting in an infinite loop.
-      return unless self == Event
-
-      action = attrs.with_indifferent_access[inheritance_column].to_i
-
-      PushEvent if action == PUSHED
-    end
-
     # Update Gitlab::ContributionsCalendar#activity_dates if this changes
     def contributions
       where("action = ? OR (target_type IN (?) AND action IN (?)) OR (target_type = ? AND action = ?)",
diff --git a/app/models/service.rb b/app/models/service.rb
index 5b8bf6e7cf0..9dcb0aab0a3 100644
--- a/app/models/service.rb
+++ b/app/models/service.rb
@@ -210,11 +210,7 @@ class Service < ActiveRecord::Base
       class_eval %{
         def #{arg}?
           # '!!' is used because nil or empty string is converted to nil
-          if Gitlab.rails5?
-            !!ActiveRecord::Type::Boolean.new.cast(#{arg})
-          else
-            !!ActiveRecord::Type::Boolean.new.type_cast_from_database(#{arg})
-          end
+          !!ActiveRecord::Type::Boolean.new.cast(#{arg})
         end
       }
     end
diff --git a/changelogs/unreleased/remove-rails4-specific-code.yml b/changelogs/unreleased/remove-rails4-specific-code.yml
new file mode 100644
index 00000000000..c6c4c0a5d5b
--- /dev/null
+++ b/changelogs/unreleased/remove-rails4-specific-code.yml
@@ -0,0 +1,5 @@
+---
+title: Remove rails4 specific code
+merge_request: 23847
+author: Jasper Maes
+type: other
diff --git a/config/application.rb b/config/application.rb
index 720196b945e..349c7258852 100644
--- a/config/application.rb
+++ b/config/application.rb
@@ -5,12 +5,6 @@ require 'rails/all'
 Bundler.require(:default, Rails.env)
 
 module Gitlab
-  # This method is used for smooth upgrading from the current Rails 4.x to Rails 5.0.
-  # https://gitlab.com/gitlab-org/gitlab-ce/issues/14286
-  def self.rails5?
-    true
-  end
-
   class Application < Rails::Application
     require_dependency Rails.root.join('lib/gitlab/redis/wrapper')
     require_dependency Rails.root.join('lib/gitlab/redis/cache')
diff --git a/db/migrate/20160226114608_add_trigram_indexes_for_searching.rb b/db/migrate/20160226114608_add_trigram_indexes_for_searching.rb
index 82b54c552e0..af8b08c095a 100644
--- a/db/migrate/20160226114608_add_trigram_indexes_for_searching.rb
+++ b/db/migrate/20160226114608_add_trigram_indexes_for_searching.rb
@@ -37,12 +37,7 @@ class AddTrigramIndexesForSearching < ActiveRecord::Migration[4.2]
     res = execute("SELECT true AS enabled FROM pg_available_extensions WHERE name = 'pg_trgm' AND installed_version IS NOT NULL;")
     row = res.first
 
-    check = if Gitlab.rails5?
-              true
-            else
-              't'
-            end
-    row && row['enabled'] == check ? true : false
+    row && row['enabled'] == true
   end
 
   def create_trigrams_extension
diff --git a/db/migrate/20161207231620_fixup_environment_name_uniqueness.rb b/db/migrate/20161207231620_fixup_environment_name_uniqueness.rb
index 7cae09021cd..420f0ccb45c 100644
--- a/db/migrate/20161207231620_fixup_environment_name_uniqueness.rb
+++ b/db/migrate/20161207231620_fixup_environment_name_uniqueness.rb
@@ -1,5 +1,4 @@
 class FixupEnvironmentNameUniqueness < ActiveRecord::Migration[4.2]
-  include Gitlab::Database::ArelMethods
   include Gitlab::Database::MigrationHelpers
 
   DOWNTIME = true
@@ -42,7 +41,7 @@ class FixupEnvironmentNameUniqueness < ActiveRecord::Migration[4.2]
 
     conflicts.each do |id, name|
       update_sql =
-        arel_update_manager
+        Arel::UpdateManager.new
           .table(environments)
           .set(environments[:name] => name + "-" + id.to_s)
           .where(environments[:id].eq(id))
diff --git a/db/migrate/20161207231626_add_environment_slug.rb b/db/migrate/20161207231626_add_environment_slug.rb
index 4657b023dfa..993b9bd3330 100644
--- a/db/migrate/20161207231626_add_environment_slug.rb
+++ b/db/migrate/20161207231626_add_environment_slug.rb
@@ -2,7 +2,6 @@
 # for more information on how to write migrations for GitLab.
 
 class AddEnvironmentSlug < ActiveRecord::Migration[4.2]
-  include Gitlab::Database::ArelMethods
   include Gitlab::Database::MigrationHelpers
 
   DOWNTIME = true
@@ -20,7 +19,7 @@ class AddEnvironmentSlug < ActiveRecord::Migration[4.2]
     finder = environments.project(:id, :name)
 
     connection.exec_query(finder.to_sql).rows.each do |id, name|
-      updater = arel_update_manager
+      updater = Arel::UpdateManager.new
         .table(environments)
         .set(environments[:slug] => generate_slug(name))
         .where(environments[:id].eq(id))
diff --git a/db/post_migrate/20161109150329_fix_project_records_with_invalid_visibility.rb b/db/post_migrate/20161109150329_fix_project_records_with_invalid_visibility.rb
index d77a22bfb69..6d9c7e4ed72 100644
--- a/db/post_migrate/20161109150329_fix_project_records_with_invalid_visibility.rb
+++ b/db/post_migrate/20161109150329_fix_project_records_with_invalid_visibility.rb
@@ -1,5 +1,4 @@
 class FixProjectRecordsWithInvalidVisibility < ActiveRecord::Migration[4.2]
-  include Gitlab::Database::ArelMethods
   include Gitlab::Database::MigrationHelpers
 
   BATCH_SIZE = 500
@@ -34,7 +33,7 @@ class FixProjectRecordsWithInvalidVisibility < ActiveRecord::Migration[4.2]
       end
 
       updates.each do |visibility_level, project_ids|
-        updater = arel_update_manager
+        updater = Arel::UpdateManager.new
           .table(projects)
           .set(projects[:visibility_level] => visibility_level)
           .where(projects[:id].in(project_ids))
diff --git a/db/post_migrate/20170324160416_migrate_user_activities_to_users_last_activity_on.rb b/db/post_migrate/20170324160416_migrate_user_activities_to_users_last_activity_on.rb
index 73989339ad9..61a5c364b2f 100644
--- a/db/post_migrate/20170324160416_migrate_user_activities_to_users_last_activity_on.rb
+++ b/db/post_migrate/20170324160416_migrate_user_activities_to_users_last_activity_on.rb
@@ -1,6 +1,5 @@
 # rubocop:disable Migration/UpdateLargeTable
 class MigrateUserActivitiesToUsersLastActivityOn < ActiveRecord::Migration[4.2]
-  include Gitlab::Database::ArelMethods
   include Gitlab::Database::MigrationHelpers
 
   disable_ddl_transaction!
@@ -40,7 +39,7 @@ class MigrateUserActivitiesToUsersLastActivityOn < ActiveRecord::Migration[4.2]
     activities = activities(day.at_beginning_of_day, day.at_end_of_day, page: page)
 
     update_sql =
-      arel_update_manager
+      Arel::UpdateManager.new
         .table(users_table)
         .set(users_table[:last_activity_on] => day.to_date)
         .where(users_table[:username].in(activities.map(&:first)))
diff --git a/lib/api/helpers.rb b/lib/api/helpers.rb
index 2cceb2ec798..a7d67a6300e 100644
--- a/lib/api/helpers.rb
+++ b/lib/api/helpers.rb
@@ -291,7 +291,7 @@ module API
         end
       end
       permitted_attrs = ActionController::Parameters.new(attrs).permit!
-      Gitlab.rails5? ? permitted_attrs.to_h : permitted_attrs
+      permitted_attrs.to_h
     end
 
     # rubocop: disable CodeReuse/ActiveRecord
diff --git a/lib/declarative_policy.rb b/lib/declarative_policy.rb
index 5e22523e45a..7ba48ae9c79 100644
--- a/lib/declarative_policy.rb
+++ b/lib/declarative_policy.rb
@@ -22,14 +22,10 @@ module DeclarativePolicy
       key = Cache.policy_key(user, subject)
 
       cache[key] ||=
-        if Gitlab.rails5?
-          # to avoid deadlocks in multi-threaded environment when
-          # autoloading is enabled, we allow concurrent loads,
-          # https://gitlab.com/gitlab-org/gitlab-ce/issues/48263
-          ActiveSupport::Dependencies.interlock.permit_concurrent_loads do
-            class_for(subject).new(user, subject, opts)
-          end
-        else
+        # to avoid deadlocks in multi-threaded environment when
+        # autoloading is enabled, we allow concurrent loads,
+        # https://gitlab.com/gitlab-org/gitlab-ce/issues/48263
+        ActiveSupport::Dependencies.interlock.permit_concurrent_loads do
           class_for(subject).new(user, subject, opts)
         end
     end
diff --git a/lib/gitlab/database.rb b/lib/gitlab/database.rb
index 6d40e00c035..b6ca777e029 100644
--- a/lib/gitlab/database.rb
+++ b/lib/gitlab/database.rb
@@ -232,11 +232,7 @@ module Gitlab
     end
 
     def self.cached_table_exists?(table_name)
-      if Gitlab.rails5?
-        connection.schema_cache.data_source_exists?(table_name)
-      else
-        connection.schema_cache.table_exists?(table_name)
-      end
+      connection.schema_cache.data_source_exists?(table_name)
     end
 
     private_class_method :connection
diff --git a/lib/gitlab/database/arel_methods.rb b/lib/gitlab/database/arel_methods.rb
deleted file mode 100644
index 991e4152dcb..00000000000
--- a/lib/gitlab/database/arel_methods.rb
+++ /dev/null
@@ -1,20 +0,0 @@
-# frozen_string_literal: true
-
-module Gitlab
-  module Database
-    module ArelMethods
-      private
-
-      # In Arel 7.0.0 (Arel 7.1.4 is used in Rails 5.0) the `engine` parameter of `Arel::UpdateManager#initializer`
-      # was removed.
-      # Remove this file and inline this method when removing rails5? code.
-      def arel_update_manager
-        if Gitlab.rails5?
-          Arel::UpdateManager.new
-        else
-          Arel::UpdateManager.new(ActiveRecord::Base)
-        end
-      end
-    end
-  end
-end
diff --git a/lib/gitlab/database/median.rb b/lib/gitlab/database/median.rb
index 0da5119a3ed..1455e410d4b 100644
--- a/lib/gitlab/database/median.rb
+++ b/lib/gitlab/database/median.rb
@@ -35,13 +35,7 @@ module Gitlab
       end
 
       def mysql_median_datetime_sql(arel_table, query_so_far, column_sym)
-        arel_from = if Gitlab.rails5?
-                      arel_table.from
-                    else
-                      arel_table
-                    end
-
-        query = arel_from
+        query = arel_table.from
                 .from(arel_table.project(Arel.sql('*')).order(arel_table[column_sym]).as(arel_table.table_name))
                 .project(average([arel_table[column_sym]], 'median'))
                 .where(
@@ -151,13 +145,8 @@ module Gitlab
                 .order(arel_table[column_sym])
             ).as('row_id')
 
-          arel_from = if Gitlab.rails5?
-                        arel_table.from.from(arel_table.alias)
-                      else
-                        arel_table.from(arel_table.alias)
-                      end
-
-          count = arel_from.project('COUNT(*)')
+          count = arel_table.from.from(arel_table.alias)
+                    .project('COUNT(*)')
                     .where(arel_table[partition_column].eq(arel_table.alias[partition_column]))
                     .as('ct')
 
diff --git a/lib/gitlab/database/migration_helpers.rb b/lib/gitlab/database/migration_helpers.rb
index d9578852db6..3abd0600e9d 100644
--- a/lib/gitlab/database/migration_helpers.rb
+++ b/lib/gitlab/database/migration_helpers.rb
@@ -3,8 +3,6 @@
 module Gitlab
   module Database
     module MigrationHelpers
-      include Gitlab::Database::ArelMethods
-
       BACKGROUND_MIGRATION_BATCH_SIZE = 1000 # Number of rows to process per job
       BACKGROUND_MIGRATION_JOB_BUFFER_SIZE = 1000 # Number of jobs to bulk queue at a time
 
@@ -361,7 +359,7 @@ module Gitlab
           stop_arel = yield table, stop_arel if block_given?
           stop_row = exec_query(stop_arel.to_sql).to_hash.first
 
-          update_arel = arel_update_manager
+          update_arel = Arel::UpdateManager.new
             .table(table)
             .set([[table[column], value]])
             .where(table[:id].gteq(start_id))
diff --git a/lib/gitlab/database/rename_reserved_paths_migration/v1/rename_base.rb b/lib/gitlab/database/rename_reserved_paths_migration/v1/rename_base.rb
index a5b42bbfdd9..60afa4bcd52 100644
--- a/lib/gitlab/database/rename_reserved_paths_migration/v1/rename_base.rb
+++ b/lib/gitlab/database/rename_reserved_paths_migration/v1/rename_base.rb
@@ -5,8 +5,6 @@ module Gitlab
     module RenameReservedPathsMigration
       module V1
         class RenameBase
-          include Gitlab::Database::ArelMethods
-
           attr_reader :paths, :migration
 
           delegate :update_column_in_batches,
@@ -66,7 +64,7 @@ module Gitlab
                                             old_full_path,
                                             new_full_path)
 
-            update = arel_update_manager
+            update = Arel::UpdateManager.new
               .table(routes)
               .set([[routes[:path], replace_statement]])
               .where(Arel::Nodes::SqlLiteral.new(filter))
diff --git a/lib/gitlab/database/sha_attribute.rb b/lib/gitlab/database/sha_attribute.rb
index 6516d6e648d..8d97adaff99 100644
--- a/lib/gitlab/database/sha_attribute.rb
+++ b/lib/gitlab/database/sha_attribute.rb
@@ -8,14 +8,7 @@ module Gitlab
         # behaviour from the default Binary type.
         ActiveRecord::ConnectionAdapters::PostgreSQL::OID::Bytea
       else
-        # In Rails 5.0 `Type` has been moved from `ActiveRecord` to `ActiveModel`
-        # https://github.com/rails/rails/commit/9cc8c6f3730df3d94c81a55be9ee1b7b4ffd29f6#diff-f8ba7983a51d687976e115adcd95822b
-        # Remove this method and leave just `ActiveModel::Type::Binary` when removing Gitlab.rails5? code.
-        if Gitlab.rails5?
-          ActiveModel::Type::Binary
-        else
-          ActiveRecord::Type::Binary
-        end
+        ActiveModel::Type::Binary
       end
 
     # Class for casting binary data to hexadecimal SHA1 hashes (and vice-versa).
@@ -26,31 +19,9 @@ module Gitlab
     class ShaAttribute < BINARY_TYPE
       PACK_FORMAT = 'H*'.freeze
 
-      # It is called from activerecord-4.2.10/lib/active_record internal methods.
-      # Remove this method when removing Gitlab.rails5? code.
-      def type_cast_from_database(value)
-        unpack_sha(super)
-      end
-
-      # It is called from activerecord-4.2.10/lib/active_record internal methods.
-      # Remove this method when removing Gitlab.rails5? code.
-      def type_cast_for_database(value)
-        serialize(value)
-      end
-
-      # It is called from activerecord-5.0.6/lib/active_record/attribute.rb
-      # Remove this method when removing Gitlab.rails5? code..
-      def deserialize(value)
-        value = Gitlab.rails5? ? super : method(:type_cast_from_database).super_method.call(value)
-
-        unpack_sha(value)
-      end
-
-      # Rename this method to `deserialize(value)` removing Gitlab.rails5? code.
       # Casts binary data to a SHA1 in hexadecimal.
-      def unpack_sha(value)
-        # Uncomment this line when removing Gitlab.rails5? code.
-        # value = super
+      def deserialize(value)
+        value = super(value)
         value ? value.unpack(PACK_FORMAT)[0] : nil
       end
 
@@ -58,7 +29,7 @@ module Gitlab
       def serialize(value)
         arg = value ? [value].pack(PACK_FORMAT) : nil
 
-        Gitlab.rails5? ? super(arg) : method(:type_cast_for_database).super_method.call(arg)
+        super(arg)
       end
     end
   end
diff --git a/lib/gitlab/database/subquery.rb b/lib/gitlab/database/subquery.rb
index 36e4559b554..10971d2b274 100644
--- a/lib/gitlab/database/subquery.rb
+++ b/lib/gitlab/database/subquery.rb
@@ -6,15 +6,11 @@ module Gitlab
       class << self
         def self_join(relation)
           t = relation.arel_table
-          t2 = if !Gitlab.rails5?
-                 relation.arel.as('t2')
-               else
-                 # Work around a bug in Rails 5, where LIMIT causes trouble
-                 # See https://gitlab.com/gitlab-org/gitlab-ce/issues/51729
-                 r = relation.limit(nil).arel
-                 r.take(relation.limit_value) if relation.limit_value
-                 r.as('t2')
-               end
+          # Work around a bug in Rails 5, where LIMIT causes trouble
+          # See https://gitlab.com/gitlab-org/gitlab-ce/issues/51729
+          r = relation.limit(nil).arel
+          r.take(relation.limit_value) if relation.limit_value
+          t2 = r.as('t2')
 
           relation.unscoped.joins(t.join(t2).on(t[:id].eq(t2[:id])).join_sources.first)
         end
diff --git a/lib/gitlab/gpg.rb b/lib/gitlab/gpg.rb
index e53c2d00743..32f61b1d65c 100644
--- a/lib/gitlab/gpg.rb
+++ b/lib/gitlab/gpg.rb
@@ -73,13 +73,7 @@ module Gitlab
       if MUTEX.locked? && MUTEX.owned?
         optimistic_using_tmp_keychain(&block)
       else
-        if Gitlab.rails5?
-          ActiveSupport::Dependencies.interlock.permit_concurrent_loads do
-            MUTEX.synchronize do
-              optimistic_using_tmp_keychain(&block)
-            end
-          end
-        else
+        ActiveSupport::Dependencies.interlock.permit_concurrent_loads do
           MUTEX.synchronize do
             optimistic_using_tmp_keychain(&block)
           end
diff --git a/lib/gitlab/middleware/correlation_id.rb b/lib/gitlab/middleware/correlation_id.rb
index 73542dd422e..80dddc41c12 100644
--- a/lib/gitlab/middleware/correlation_id.rb
+++ b/lib/gitlab/middleware/correlation_id.rb
@@ -20,11 +20,7 @@ module Gitlab
       private
 
       def correlation_id(env)
-        if Gitlab.rails5?
-          request(env).request_id
-        else
-          request(env).uuid
-        end
+        request(env).request_id
       end
 
       def request(env)
diff --git a/lib/mysql_zero_date.rb b/lib/mysql_zero_date.rb
index 216560148fa..f36610abf8f 100644
--- a/lib/mysql_zero_date.rb
+++ b/lib/mysql_zero_date.rb
@@ -17,4 +17,4 @@ module MysqlZeroDate
   end
 end
 
-ActiveRecord::ConnectionAdapters::AbstractMysqlAdapter.prepend(MysqlZeroDate) if Gitlab.rails5?
+ActiveRecord::ConnectionAdapters::AbstractMysqlAdapter.prepend(MysqlZeroDate)
diff --git a/spec/controllers/application_controller_spec.rb b/spec/controllers/application_controller_spec.rb
index c2bd7fd9808..945c9f97fc3 100644
--- a/spec/controllers/application_controller_spec.rb
+++ b/spec/controllers/application_controller_spec.rb
@@ -551,14 +551,7 @@ describe ApplicationController do
       subject { get :index, text: "hi \255" }
 
       it 'renders 412' do
-        if Gitlab.rails5?
-          expect { subject }.to raise_error(ActionController::BadRequest)
-        else
-          subject
-
-          expect(response).to have_gitlab_http_status(412)
-          expect(response).to render_template :precondition_failed
-        end
+        expect { subject }.to raise_error(ActionController::BadRequest)
       end
     end
 
@@ -566,16 +559,7 @@ describe ApplicationController do
       subject { get :index, text: "hi \255", format: :js }
 
       it 'renders 412' do
-        if Gitlab.rails5?
-          expect { subject }.to raise_error(ActionController::BadRequest)
-        else
-          subject
-
-          json_response = JSON.parse(response.body)
-
-          expect(response).to have_gitlab_http_status(412)
-          expect(json_response['error']).to eq('Invalid UTF-8')
-        end
+        expect { subject }.to raise_error(ActionController::BadRequest)
       end
     end
   end
diff --git a/spec/controllers/boards/lists_controller_spec.rb b/spec/controllers/boards/lists_controller_spec.rb
index 80631d2efb0..16ccf405247 100644
--- a/spec/controllers/boards/lists_controller_spec.rb
+++ b/spec/controllers/boards/lists_controller_spec.rb
@@ -163,11 +163,7 @@ describe Boards::ListsController do
                  list: { position: position },
                  format: :json }
 
-      if Gitlab.rails5?
-        patch :update, params: params, as: :json
-      else
-        patch :update, params
-      end
+      patch :update, params: params, as: :json
     end
   end
 
diff --git a/spec/controllers/projects/merge_requests_controller_spec.rb b/spec/controllers/projects/merge_requests_controller_spec.rb
index a37a831ddbb..e837c99d19c 100644
--- a/spec/controllers/projects/merge_requests_controller_spec.rb
+++ b/spec/controllers/projects/merge_requests_controller_spec.rb
@@ -357,11 +357,7 @@ describe Projects::MergeRequestsController do
     context 'when the sha parameter matches the source SHA' do
       def merge_with_sha(params = {})
         post_params = base_params.merge(sha: merge_request.diff_head_sha).merge(params)
-        if Gitlab.rails5?
-          post :merge, params: post_params, as: :json
-        else
-          post :merge, post_params
-        end
+        post :merge, params: post_params, as: :json
       end
 
       it 'returns :success' do
diff --git a/spec/controllers/projects/pipeline_schedules_controller_spec.rb b/spec/controllers/projects/pipeline_schedules_controller_spec.rb
index 7179423dde2..08e2c957d69 100644
--- a/spec/controllers/projects/pipeline_schedules_controller_spec.rb
+++ b/spec/controllers/projects/pipeline_schedules_controller_spec.rb
@@ -310,19 +310,11 @@ describe Projects::PipelineSchedulesController do
     end
 
     def go
-      if Gitlab.rails5?
-        put :update, params: { namespace_id: project.namespace.to_param,
-                               project_id: project,
-                               id: pipeline_schedule,
-                               schedule: schedule },
-                     as: :html
-
-      else
-        put :update, namespace_id: project.namespace.to_param,
-                     project_id: project,
-                     id: pipeline_schedule,
-                     schedule: schedule
-      end
+      put :update, params: { namespace_id: project.namespace.to_param,
+                             project_id: project,
+                             id: pipeline_schedule,
+                             schedule: schedule },
+                   as: :html
     end
   end
 
diff --git a/spec/controllers/uploads_controller_spec.rb b/spec/controllers/uploads_controller_spec.rb
index 6420b70a54f..832649e5886 100644
--- a/spec/controllers/uploads_controller_spec.rb
+++ b/spec/controllers/uploads_controller_spec.rb
@@ -8,11 +8,7 @@ end
 shared_examples 'content not cached without revalidation and no-store' do
   it 'ensures content will not be cached without revalidation' do
     # Fixed in newer versions of ActivePack, it will only output a single `private`.
-    if Gitlab.rails5?
-      expect(subject['Cache-Control']).to eq('max-age=0, private, must-revalidate, no-store')
-    else
-      expect(subject['Cache-Control']).to eq('max-age=0, private, must-revalidate, private, no-store')
-    end
+    expect(subject['Cache-Control']).to eq('max-age=0, private, must-revalidate, no-store')
   end
 end
 
diff --git a/spec/helpers/storage_helper_spec.rb b/spec/helpers/storage_helper_spec.rb
index c580b78c908..03df9deafa1 100644
--- a/spec/helpers/storage_helper_spec.rb
+++ b/spec/helpers/storage_helper_spec.rb
@@ -15,11 +15,7 @@ describe StorageHelper do
     end
 
     it "uses commas as thousands separator" do
-      if Gitlab.rails5?
-        expect(helper.storage_counter(100_000_000_000_000_000_000_000)).to eq("86,736.2 EB")
-      else
-        expect(helper.storage_counter(100_000_000_000_000_000)).to eq("90,949.5 TB")
-      end
+      expect(helper.storage_counter(100_000_000_000_000_000_000_000)).to eq("86,736.2 EB")
     end
   end
 end
diff --git a/spec/lib/gitlab/database_spec.rb b/spec/lib/gitlab/database_spec.rb
index 0826bc3eeed..60106ee3c0b 100644
--- a/spec/lib/gitlab/database_spec.rb
+++ b/spec/lib/gitlab/database_spec.rb
@@ -400,13 +400,8 @@ describe Gitlab::Database do
 
   describe '.cached_table_exists?' do
     it 'only retrieves data once per table' do
-      if Gitlab.rails5?
-        expect(ActiveRecord::Base.connection).to receive(:data_source_exists?).with(:projects).once.and_call_original
-        expect(ActiveRecord::Base.connection).to receive(:data_source_exists?).with(:bogus_table_name).once.and_call_original
-      else
-        expect(ActiveRecord::Base.connection).to receive(:table_exists?).with(:projects).once.and_call_original
-        expect(ActiveRecord::Base.connection).to receive(:table_exists?).with(:bogus_table_name).once.and_call_original
-      end
+      expect(ActiveRecord::Base.connection).to receive(:data_source_exists?).with(:projects).once.and_call_original
+      expect(ActiveRecord::Base.connection).to receive(:data_source_exists?).with(:bogus_table_name).once.and_call_original
 
       2.times do
         expect(described_class.cached_table_exists?(:projects)).to be_truthy
diff --git a/spec/lib/gitlab/sql/glob_spec.rb b/spec/lib/gitlab/sql/glob_spec.rb
index 1cf8935bfe3..3147b52dcc5 100644
--- a/spec/lib/gitlab/sql/glob_spec.rb
+++ b/spec/lib/gitlab/sql/glob_spec.rb
@@ -35,9 +35,8 @@ describe Gitlab::SQL::Glob do
     value = query("SELECT #{quote(string)} LIKE #{pattern}")
               .rows.flatten.first
 
-    check = Gitlab.rails5? ? true : 't'
     case value
-    when check, 1
+    when true, 1
       true
     else
       false
diff --git a/spec/models/notification_setting_spec.rb b/spec/models/notification_setting_spec.rb
index 771d834c4bc..c8ab564e3bc 100644
--- a/spec/models/notification_setting_spec.rb
+++ b/spec/models/notification_setting_spec.rb
@@ -42,12 +42,7 @@ RSpec.describe NotificationSetting do
         expect(notification_setting.new_issue).to eq(true)
         expect(notification_setting.close_issue).to eq(true)
         expect(notification_setting.merge_merge_request).to eq(true)
-
-        # In Rails 5 assigning a value which is not explicitly `true` or `false` ("nil" in this case)
-        # to a boolean column transforms it to `true`.
-        # In Rails 4 it transforms the value to `false` with deprecation warning.
-        # Replace `eq(Gitlab.rails5?)` with `eq(true)` when removing rails5? code.
-        expect(notification_setting.close_merge_request).to eq(Gitlab.rails5?)
+        expect(notification_setting.close_merge_request).to eq(true)
         expect(notification_setting.reopen_merge_request).to eq(false)
       end
     end
diff --git a/spec/requests/api/internal_spec.rb b/spec/requests/api/internal_spec.rb
index 2ebcb787d06..3304dfdd909 100644
--- a/spec/requests/api/internal_spec.rb
+++ b/spec/requests/api/internal_spec.rb
@@ -969,17 +969,10 @@ describe API::Internal do
       env: env
     }
 
-    if Gitlab.rails5?
-      post(
-        api("/internal/allowed"),
-        params: params
-      )
-    else
-      post(
-        api("/internal/allowed"),
-        params
-      )
-    end
+    post(
+      api("/internal/allowed"),
+      params: params
+    )
   end
 
   def archive(key, project)
diff --git a/spec/support/helpers/test_request_helpers.rb b/spec/support/helpers/test_request_helpers.rb
index 187a0e07891..5a84d67bdfc 100644
--- a/spec/support/helpers/test_request_helpers.rb
+++ b/spec/support/helpers/test_request_helpers.rb
@@ -2,10 +2,6 @@
 
 module TestRequestHelpers
   def test_request(remote_ip: '127.0.0.1')
-    if Gitlab.rails5?
-      ActionController::TestRequest.new({ remote_ip: remote_ip }, ActionController::TestSession.new)
-    else
-      ActionController::TestRequest.new(remote_ip: remote_ip)
-    end
+    ActionController::TestRequest.new({ remote_ip: remote_ip }, ActionController::TestSession.new)
   end
 end