diff --git a/README.md b/README.md
index 0ab77de..c9f5d8e 100644
--- a/README.md
+++ b/README.md
@@ -148,6 +148,14 @@ class Account < ApplicationRecord
 end
 ```
 
+Custom index can be handled with a Hash containing a literal key :
+
+```ruby
+class Account < ApplicationRecord
+  upsert_keys literal: 'md5(my_long_field)'
+end
+```
+
 ## Tests
 
 Make sure to have an upsert_test database:
diff --git a/lib/active_record_upsert/active_record/persistence.rb b/lib/active_record_upsert/active_record/persistence.rb
index ca91a38..4e65e75 100644
--- a/lib/active_record_upsert/active_record/persistence.rb
+++ b/lib/active_record_upsert/active_record/persistence.rb
@@ -75,7 +75,7 @@ def upsert_keys(*keys)
           options = keys.extract_options!
           keys = keys.first if keys.size == 1 # support single string/symbol, multiple string/symbols, and array
           return if keys.nil?
-          @_upsert_keys = Array(keys).map(&:to_s)
+          @_upsert_keys = Array(keys)
           @_upsert_options = options
         end
 
diff --git a/lib/active_record_upsert/arel/crud.rb b/lib/active_record_upsert/arel/crud.rb
index ce030c7..1d831c9 100644
--- a/lib/active_record_upsert/arel/crud.rb
+++ b/lib/active_record_upsert/arel/crud.rb
@@ -12,9 +12,11 @@
 module Arel
   module Crud
     def compile_upsert(upsert_keys, upsert_options, upsert_values, insert_values, wheres)
+      # Support non-attribute key (like `md5(my_attribute)``)
+      target = self[upsert_keys.kind_of?(Hash) ? ::Arel::Nodes::SqlLiteral.new(upsert_keys[:literal]) : upsert_keys.join(',')]
       on_conflict_do_update = OnConflictDoUpdateManager.new
 
-      on_conflict_do_update.target = self[upsert_keys.join(',')]
+      on_conflict_do_update.target = target
       on_conflict_do_update.target_condition = upsert_options[:where]
       on_conflict_do_update.wheres = wheres
       on_conflict_do_update.set(upsert_values)
diff --git a/spec/active_record/key_spec.rb b/spec/active_record/key_spec.rb
index a798b50..2e01ba7 100644
--- a/spec/active_record/key_spec.rb
+++ b/spec/active_record/key_spec.rb
@@ -43,8 +43,9 @@ module ActiveRecord
         end
       end
       context 'different ways of setting keys' do
-        let(:attrs) { {make: 'Ford', name: 'Focus'} }
+        let(:attrs) { {make: 'Ford', name: 'Focus', long_field: SecureRandom.uuid} }
         before { Vehicle.create(attrs) }
+
         it 'works with multiple symbol args' do
           Vehicle.upsert_keys :make, :name
           upserted = Vehicle.new(**attrs, wheels_count: 1)
@@ -85,6 +86,14 @@ module ActiveRecord
           expect(upserted.wheels_count).to eq(1)
           expect(upserted.id).to eq(v.id)
         end
+        it 'works with a literal' do
+          v = Vehicle.create
+          Vehicle.upsert_keys literal: 'md5(long_field)'
+          upserted = Vehicle.new(id: v.id, long_field: attrs[:long_field])
+          upserted.upsert
+          expect(upserted.long_field).to eq(attrs[:long_field])
+          expect(upserted.id).to eq(v.id)
+        end
       end
 
       context 'when the record is not new' do
diff --git a/spec/dummy/db/migrate/20160419103547_create_vehicles.rb b/spec/dummy/db/migrate/20160419103547_create_vehicles.rb
index d5e35ac..4d493e4 100644
--- a/spec/dummy/db/migrate/20160419103547_create_vehicles.rb
+++ b/spec/dummy/db/migrate/20160419103547_create_vehicles.rb
@@ -4,10 +4,12 @@ def change
       t.integer :wheels_count
       t.string :name
       t.string :make
+      t.string :long_field
 
       t.timestamps
 
       t.index [:make, :name], unique: true
+      t.index 'md5(long_field)', unique: true
     end
   end
 end
diff --git a/spec/dummy/db/structure.sql b/spec/dummy/db/structure.sql
index f3aa339..3ae40ee 100644
--- a/spec/dummy/db/structure.sql
+++ b/spec/dummy/db/structure.sql
@@ -45,6 +45,7 @@ CREATE TABLE accounts (
 --
 
 CREATE SEQUENCE accounts_id_seq
+    AS integer
     START WITH 1
     INCREMENT BY 1
     NO MINVALUE
@@ -89,6 +90,7 @@ CREATE TABLE my_records (
 --
 
 CREATE SEQUENCE my_records_id_seq
+    AS integer
     START WITH 1
     INCREMENT BY 1
     NO MINVALUE
@@ -121,6 +123,7 @@ CREATE TABLE vehicles (
     wheels_count integer,
     name character varying,
     make character varying,
+    long_field character varying,
     created_at timestamp without time zone NOT NULL,
     updated_at timestamp without time zone NOT NULL
 );
@@ -131,6 +134,7 @@ CREATE TABLE vehicles (
 --
 
 CREATE SEQUENCE vehicles_id_seq
+    AS integer
     START WITH 1
     INCREMENT BY 1
     NO MINVALUE
@@ -227,6 +231,13 @@ CREATE UNIQUE INDEX index_my_records_on_wisdom ON my_records USING btree (wisdom
 CREATE UNIQUE INDEX index_vehicles_on_make_and_name ON vehicles USING btree (make, name);
 
 
+--
+-- Name: index_vehicles_on_md5_long_field; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE UNIQUE INDEX index_vehicles_on_md5_long_field ON vehicles USING btree (md5((long_field)::text));
+
+
 --
 -- PostgreSQL database dump complete
 --
@@ -237,3 +248,5 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20160419103547'),
 ('20160419124138'),
 ('20160419124140');
+
+
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 9b77bce..503c531 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -1,6 +1,7 @@
 $LOAD_PATH.unshift File.expand_path('../../lib', __FILE__)
 require 'active_record'
 require 'database_cleaner'
+require 'securerandom'
 
 # Configure Rails Environment
 ENV['RAILS_ENV'] = 'test'