README.md in postgres_upsert-4.0.0 vs README.md in postgres_upsert-5.0.0

@@ -26,11 +26,11 @@
 <io_object_or_file_path> can be either a string representing a file path, or an io object (StringIO, FileIO, etc.)
 
 options:
 - :delimiter - the string to use to delimit fields from the source data.  Default is ","
 - :header => specifies if the file/io source contains a header row.  Either :header option must be true, or :columns list must be passed.  Default true
-- :key_column => the primary key or unique key column on your destination table, used to distinguish new records from existing records.  Default is the primary_key of your destination table/model.
+- :unique_key => the primary key or unique key column (or composite key columns) on your destination table, used to distinguish new records from existing records.  Default is the primary_key of your destination table/model.
 - :update_only => when true, postgres_upsert will ONLY update existing records, and not insert new.  Default is false.
 
 ## Examples
 for these examples let's assume we have a users table and model:
 ```ruby
@@ -64,20 +64,27 @@
 * newly inserted records get a current timestamp for created_at
 * records existing in the source file/IO will get an update to their updated_at timestamp (even if all fields maintain the same value)
 * records that are in the destination table but not the source will not have their timestamps changed.
 
 
-### Overriding the key_column
+### Overriding the unique_key
 
-By default postgres_upsert uses the primary key on your ActiveRecord table to determine if each record should be inserted or updated.  You can override the column using the :key_field option:
+By default postgres_upsert uses the primary key on your ActiveRecord table to determine if each record should be inserted or updated.  You can override the column using the :unique_key option:
 
 ```ruby
-PostgresUpsert.write User "/tmp/users.csv", :key_column => ["external_twitter_id"]
+PostgresUpsert.write User "/tmp/users.csv", :unique_key => ["external_twitter_id"]
 ```
 
 obviously, the field you pass must be a unique key in your database (this is not enforced at the moment, but will be)
 
+If your source data does not contain the primary key, or an individual unique key, you can pass multiple columns in the unique_key option:
+
+```ruby
+  PostgresUpsert.write User "/tmp/users.csv", :unique_key => ["state_id_numer", "state_name"]
+```
+As long as combined columns represent a unique value, row, we can successfully upsert.
+
 passing :update_only => true will ensure that no new records are created, but records will be updated.
 
 ### Insert/Update Counts
 PostgresUpsert with also return a PostgresUpsert::Result object that will tell you how many records were inserted or updated:
 
@@ -122,10 +129,10 @@
         (1..10000).each do |n|
           csv << ["number#{n}@postgres.up", "#{(n-5..n).to_a.join('')}"]
         end
       end
       io = StringIO.new(csv_string)
-      PostgresUpsert.write User io, key_column: "email"
+      PostgresUpsert.write User io, unique_key: "email"
     end
     puts time
 end
 ```