Ensures that primary_key method will return nil when multi-pk

When table has a composite primary key, the `primary_key` method for
sqlite3 and postgresql was only returning the first field of the key.

Ensures that it will return nil instead, as AR dont support composite pks.

1 files changed, 4 insertions, 4 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
index 74b0833a7e..b2a7b40ded 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
@@ -385,15 +385,15 @@ module ActiveRecord
 
         # Returns just a table's primary key
         def primary_key(table)
-          row = exec_query(<<-end_sql, 'SCHEMA').rows.first
+          pks = exec_query(<<-end_sql, 'SCHEMA').rows
             SELECT attr.attname
             FROM pg_attribute attr
-            INNER JOIN pg_constraint cons ON attr.attrelid = cons.conrelid AND attr.attnum = cons.conkey[1]
+            INNER JOIN pg_constraint cons ON attr.attrelid = cons.conrelid AND attr.attnum = any(cons.conkey)
             WHERE cons.contype = 'p'
               AND cons.conrelid = '#{quote_table_name(table)}'::regclass
           end_sql
-
-          row && row.first
+          return nil unless pks.count == 1
+          pks[0][0]
         end
 
         # Renames a table.