Use different queries for MySQL and Postgres

I could not find a query that worked on both; these two queries look
very similar and seem to do the same thing.

db/migrate/20150411000035_fix_identities.rb

@@ -17,7 +17,20 @@ class FixIdentities < ActiveRecord::Migration
                    end
 
     # Delete duplicate identities
-    execute "DELETE FROM identities WHERE provider = 'ldap' AND user_id IN (SELECT user_id FROM identities WHERE provider = '#{new_provider}')"
+    # We use a sort of self-join to find rows in identities which match on
+    # user_id but where one has provider 'ldap'. We delete the duplicate row
+    # with provider 'ldap'.
+    delete_statement = ''
+    case adapter_name.downcase
+    when /^mysql/
+      delete_statement << 'DELETE FROM id1 USING identities AS id1, identities AS id2'
+    when 'postgresql'
+      delete_statement << 'DELETE FROM identities AS id1 USING identities AS id2'
+    else
+      raise "Unknown DB adapter: #{adapter_name}"
+    end
+    delete_statement << " WHERE id1.user_id = id2.user_id AND id1.provider = 'ldap' AND id2.provider = '#{new_provider}'"
+    execute delete_statement
 
     # Update legacy identities
     execute "UPDATE identities SET provider = '#{new_provider}' WHERE provider = 'ldap';"