diff --git a/.rubocop.yml b/.rubocop.yml
index e1d6acf7e..037ef6a9a 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -101,6 +101,7 @@ Style/CharacterLiteral:
 Style/ClassAndModuleChildren:
   Enabled: false
 Style/CollectionMethods:
+  Enabled: true
   PreferredMethods:
     find: detect
     reduce: inject
diff --git a/Appraisals b/Appraisals
index e76d3358e..13c3472c6 100644
--- a/Appraisals
+++ b/Appraisals
@@ -111,24 +111,26 @@ appraise 'rails_5_2' do
   gem 'pg', '~> 1.1', platform: :ruby
 end
 
-appraise 'rails_6_0' do
-  instance_eval(&shared_dependencies)
-
-  gem 'rails', '~> 6.0.0.beta3'
-  gem 'puma', '~> 3.11'
-  gem 'bootsnap', '>= 1.4.1', require: false
-  gem 'sass-rails', '~> 5.0'
-  gem 'webpacker', '>= 4.0.0.rc3'
-  gem 'turbolinks', '~> 5'
-  gem 'jbuilder', '~> 2.5'
-  gem 'bcrypt', '~> 3.1.7'
-  gem 'capybara', '>= 2.15'
-  gem 'listen', '>= 3.0.5', '< 3.2'
-  gem 'spring-watcher-listen', '~> 2.0.0'
-  gem 'selenium-webdriver'
-  gem 'chromedriver-helper'
-
-  # Other dependencies
-  gem 'rails-controller-testing', '>= 1.0.1'
-  gem 'pg', '~> 1.1', platform: :ruby
+if Gem::Requirement.new('>= 2.5.0').satisfied_by?(Gem::Version.new(RUBY_VERSION))
+  appraise 'rails_6_0' do
+    instance_eval(&shared_dependencies)
+
+    gem 'rails', '~> 6.0.0.beta3'
+    gem 'puma', '~> 3.11'
+    gem 'bootsnap', '>= 1.4.1', require: false
+    gem 'sass-rails', '~> 5.0'
+    gem 'webpacker', '>= 4.0.0.rc3'
+    gem 'turbolinks', '~> 5'
+    gem 'jbuilder', '~> 2.5'
+    gem 'bcrypt', '~> 3.1.7'
+    gem 'capybara', '>= 2.15'
+    gem 'listen', '>= 3.0.5', '< 3.2'
+    gem 'spring-watcher-listen', '~> 2.0.0'
+    gem 'selenium-webdriver'
+    gem 'chromedriver-helper'
+
+    # Other dependencies
+    gem 'rails-controller-testing', '>= 1.0.1'
+    gem 'pg', '~> 1.1', platform: :ruby
+  end
 end
diff --git a/bin/setup b/bin/setup
index f88f866bc..96b9c181f 100755
--- a/bin/setup
+++ b/bin/setup
@@ -3,6 +3,7 @@
 set -euo pipefail
 
 RUBY_VERSION=$(script/supported_ruby_versions | xargs -n 1 echo | sort -V | tail -n 1)
+required_ruby_version=$(cat .ruby-version)
 
 cd "$(dirname "$(dirname "$0")")"
 
@@ -30,6 +31,10 @@ error() {
   echo -e "\033[31m$@\033[0m"
 }
 
+echo-wrapped() {
+  echo "$@" | fmt -w 80 | cat
+}
+
 has-executable() {
   type "$1" &>/dev/null
 }
@@ -38,6 +43,14 @@ is-running() {
   pgrep "$1" >/dev/null
 }
 
+start() {
+  if has-executable brew; then
+    brew services start "$1"
+  else
+    sudo service "${2:-$1}" start
+  fi
+}
+
 install() {
   local apt_package=""
   local rpm_package=""
@@ -90,9 +103,10 @@ check-for-build-tools() {
   if [[ $platform == "linux" ]]; then
     if ! has-executable apt-get; then
       error "You don't seem to have a package manager installed."
-      echo "The setup script assumes you're using Debian or a Debian-derived flavor of Linux"
-      echo "(i.e. something with Apt). If this is not the case, then we would gladly take a"
-      echo "PR fixing this!"
+      echo-wrapped "\
+The setup script assumes you're using Debian or a Debian-derived flavor of
+Linux (i.e. something with Apt). If this is not the case, then we would
+gladly take a PR fixing this!"
       exit 1
     fi
 
@@ -100,10 +114,12 @@ check-for-build-tools() {
   else
     if ! has-executable brew; then
       error "You don't seem to have Homebrew installed."
-      echo
-      echo "Follow the instructions here to do this:"
-      echo
-      echo "http://brew.sh"
+      echo-wrapped "\
+Follow the instructions here to do this:
+
+    http://brew.sh
+
+Then re-run this script."
       exit 1
     fi
 
@@ -145,21 +161,22 @@ install-dependencies() {
       rbenv install --skip-existing "$RUBY_VERSION"
     fi
   elif has-executable rvm; then
-    if ! (rvm ls | grep $RUBY_VERSION'\>' &>/dev/null); then
-      banner "Installing Ruby $RUBY_VERSION with rvm"
-      error "You don't seem to have Ruby $RUBY_VERSION installed."
-      echo
-      echo "Use RVM to do so, and then re-run this command."
-      echo
+    if ! (rvm list | grep $required_ruby_version'\>' &>/dev/null); then
+      banner "Installing Ruby $required_ruby_version with rvm"
+      rvm install $required_ruby_version
+      rvm use $required_ruby_version
     fi
   else
     error "You don't seem to have a Ruby manager installed."
-    echo
-    echo 'We recommend using rbenv. You can find installation instructions here:'
-    echo
-    echo 'http://github.com/rbenv/rbenv'
-    echo
-    echo "When you're done, simply re-run this script!"
+    echo-wrapped "\
+We recommend using rbenv. You can find instructions to install it here:
+
+    https://github.com/rbenv/rbenv#installation
+
+Make sure to follow the instructions to configure your shell so that rbenv is
+automatically loaded.
+
+When you're done, open up a new terminal tab and re-run this script."
     exit 1
   fi
 
@@ -167,22 +184,6 @@ install-dependencies() {
   gem install bundler -v '~> 1.0' --conservative
   bundle check || bundle install
   bundle exec appraisal install
-
-  if ! has-executable node; then
-    banner 'Installing Node'
-
-    if [[ $platform == 'linux' ]]; then
-      curl -sL https://deb.nodesource.com/setup_10.x | sudo -E bash -
-
-      install nodejs
-
-      if ! has-executable npm; then
-        install npm
-      fi
-    else
-      install nodejs
-    fi
-  fi
 }
 
 check-for-build-tools
diff --git a/lib/shoulda/matchers/active_record/have_db_index_matcher.rb b/lib/shoulda/matchers/active_record/have_db_index_matcher.rb
index 24b8edf2b..6cfe5d85b 100644
--- a/lib/shoulda/matchers/active_record/have_db_index_matcher.rb
+++ b/lib/shoulda/matchers/active_record/have_db_index_matcher.rb
@@ -49,6 +49,30 @@ module ActiveRecord
       #       should have_db_index([:user_id, :name])
       #     end
       #
+      # Finally, if you're using Rails 5 and PostgreSQL, you can also specify an
+      # expression:
+      #
+      #     class CreateLoggedErrors < ActiveRecord::Migration
+      #       def change
+      #         create_table :logged_errors do |t|
+      #           t.string :code
+      #           t.jsonb :content
+      #         end
+      #
+      #         add_index :logged_errors, 'lower(code)::text'
+      #       end
+      #     end
+      #
+      #     # RSpec
+      #     RSpec.describe LoggedError, type: :model do
+      #       it { should have_db_index('lower(code)::text') }
+      #     end
+      #
+      #     # Minitest (Shoulda)
+      #     class LoggedErrorTest < ActiveSupport::TestCase
+      #       should have_db_index('lower(code)::text')
+      #     end
+      #
       # #### Qualifiers
       #
       # ##### unique
@@ -171,9 +195,16 @@ def correct_unique?
         end
 
         def matched_index
-          @_matched_index ||= actual_indexes.find do |index|
-            index.columns == expected_columns
-          end
+          @_matched_index ||=
+            if expected_columns.one?
+              actual_indexes.detect do |index|
+                Array.wrap(index.columns) == expected_columns
+              end
+            else
+              actual_indexes.detect do |index|
+                index.columns == expected_columns
+              end
+            end
         end
 
         def actual_indexes
diff --git a/spec/support/unit/helpers/active_record_versions.rb b/spec/support/unit/helpers/active_record_versions.rb
index 44eee019b..1d35db635 100644
--- a/spec/support/unit/helpers/active_record_versions.rb
+++ b/spec/support/unit/helpers/active_record_versions.rb
@@ -40,5 +40,9 @@ def active_record_uniqueness_supports_array_columns?
     def active_record_supports_optional_for_associations?
       active_record_version >= 5
     end
+
+    def active_record_supports_expression_indexes?
+      active_record_version >= 5
+    end
   end
 end
diff --git a/spec/support/unit/helpers/database_helpers.rb b/spec/support/unit/helpers/database_helpers.rb
index f7a267b95..df7c9f4c9 100644
--- a/spec/support/unit/helpers/database_helpers.rb
+++ b/spec/support/unit/helpers/database_helpers.rb
@@ -16,5 +16,6 @@ def postgresql?
     alias_method :database_supports_array_columns?, :postgresql?
     alias_method :database_supports_uuid_columns?, :postgresql?
     alias_method :database_supports_money_columns?, :postgresql?
+    alias_method :database_supports_expression_indexes?, :postgresql?
   end
 end
diff --git a/spec/unit/shoulda/matchers/active_record/have_db_index_matcher_spec.rb b/spec/unit/shoulda/matchers/active_record/have_db_index_matcher_spec.rb
index f19eaad46..2c1025c50 100644
--- a/spec/unit/shoulda/matchers/active_record/have_db_index_matcher_spec.rb
+++ b/spec/unit/shoulda/matchers/active_record/have_db_index_matcher_spec.rb
@@ -1,6 +1,11 @@
 require 'unit_spec_helper'
 
 describe Shoulda::Matchers::ActiveRecord::HaveDbIndexMatcher, type: :model do
+  def self.can_test_expression_indexes?
+    active_record_supports_expression_indexes? &&
+      database_supports_expression_indexes?
+  end
+
   describe 'the matcher' do
     # rubocop:disable Layout/MultilineBlockLayout
     # rubocop:disable Layout/SpaceAroundBlockParameters
@@ -237,6 +242,99 @@
           end
         end
       end
+
+      if can_test_expression_indexes?
+        context 'when given an expression' do
+          context 'qualified with nothing' do
+            context 'when the table has the given index' do
+              it 'matches when used in the positive' do
+                record = record_with_index_on(
+                  'lower((code)::text)',
+                  columns: { code: :string },
+                )
+                expect(record).to have_db_index('lower((code)::text)')
+              end
+
+              it 'does not match when used in the negative' do
+                record = record_with_index_on(
+                  'lower((code)::text)',
+                  model_name: 'Example',
+                  columns: { code: :string },
+                )
+
+                assertion = lambda do
+                  expect(record).not_to have_db_index('lower((code)::text)')
+                end
+
+                expect(&assertion).to fail_with_message(<<-MESSAGE, wrap: true)
+Expected the examples table not to have an index on "lower((code)::text)", but
+it does.
+                MESSAGE
+              end
+            end
+
+            context 'when the table does not have the given index' do
+              it 'matches when used in the negative' do
+                record = record_with_index_on(
+                  'code',
+                  columns: { code: :string },
+                )
+                expect(record).not_to have_db_index('lower((code)::text)')
+              end
+
+              it 'does not match when used in the positive' do
+                record = record_with_index_on(
+                  'code',
+                  model_name: 'Example',
+                  columns: { code: :string },
+                )
+
+                assertion = lambda do
+                  expect(record).to have_db_index('lower((code)::text)')
+                end
+
+                expect(&assertion).to fail_with_message(<<-MESSAGE, wrap: true)
+Expected the examples table to have an index on "lower((code)::text)", but it
+does not.
+                MESSAGE
+              end
+            end
+          end
+
+          context 'when qualified with unique' do
+            include_examples(
+              'for when the matcher is qualified',
+              index: 'lower((code)::text)',
+              other_index: 'code',
+              columns: { code: :string },
+              unique: true,
+              qualifier_args: [],
+            )
+          end
+
+          context 'when qualified with unique: true' do
+            include_examples(
+              'for when the matcher is qualified',
+              index: 'lower((code)::text)',
+              other_index: 'code',
+              columns: { code: :string },
+              unique: true,
+              qualifier_args: [true],
+            )
+          end
+
+          context 'when qualified with unique: false' do
+            include_examples(
+              'for when the matcher is qualified',
+              index: 'lower((code)::text)',
+              other_index: 'code',
+              columns: { code: :string },
+              unique: false,
+              qualifier_args: [false],
+            )
+          end
+        end
+      end
     end
 
     context 'when not all models are connected to the same database' do
@@ -352,6 +450,44 @@
         )
       end
     end
+
+    if can_test_expression_indexes?
+      context 'when given an expression' do
+        context 'when not qualified with anything' do
+          it 'returns the correct description' do
+            matcher = have_db_index('lower(code)')
+            expect(matcher.description).to eq('have an index on "lower(code)"')
+          end
+        end
+
+        context 'when qualified with unique' do
+          include_examples(
+            'for when the matcher is qualified',
+            index: 'lower(code)',
+            index_type: 'unique',
+            qualifier_args: [],
+          )
+        end
+
+        context 'when qualified with unique: true' do
+          include_examples(
+            'for when the matcher is qualified',
+            index: 'lower(code)',
+            index_type: 'unique',
+            qualifier_args: [true],
+          )
+        end
+
+        context 'when qualified with unique: false' do
+          include_examples(
+            'for when the matcher is qualified',
+            index: 'lower(code)',
+            index_type: 'non-unique',
+            qualifier_args: [false],
+          )
+        end
+      end
+    end
   end
 
   def record_with_index_on(