2 files changed, 7 insertions, 8 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.
diff --git a/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb b/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb
index 0f7e0fac01..67b2c6b1e6 100644
--- a/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb
@@ -418,10 +418,9 @@ module ActiveRecord
       end
 
       def primary_key(table_name) #:nodoc:
-        column = table_structure(table_name).find { |field|
-          field['pk'] == 1
-        }
-        column && column['name']
+        pks = table_structure(table_name).select { |f| f['pk'] > 0 }
+        return nil unless pks.count == 1
+        pks[0]['name']
       end
 
       def remove_index!(table_name, index_name) #:nodoc: