如何在数据库级别获取primary_key的列名？

Question

I need to get the column name containing the primary key(s) at the database level.

Model.primary_key returns models_id column name which is not the primary key at the database level. I cannot change migrations or modify the tables in anyway.

I can currently get it through the MySqlAdapter with ActiveRecord::Base.connection.primary_key('table_name')

but this does not work with tables that have composite primary keys. If the table contains a composite primary key it returns nil .

Is there any way I can achieve this programatically?

Answer 1

ActiveRecord does not support composite primary keys out of the box.

You'll need to install the composite_primary_keys gem and then ActiveRecord::Base.connection.primary_key('table_name') will return an array of column names if the primary key is composite.

Answer 2

1. Execute the query:

    SHOW COLUMNS FROM table_name; 

Then look at all the rows returned where the columns named KEY has the value 'PRI' . Then column Field will give you the column name in your table that forms part of the table's primary key.

2. Alternatively:

    SELECT COLUMN_NAME FROM information_schema WHERE TABLE_SCHEMA='database_name' AND TABLE_NAME = 'table_name' AND COLUMN_KEY = 'PRI';