laravel migration remove unique constraint

1/** To drop an index you must specify the index's name. 
2Laravel assigns a reasonable name to the indexes by default. 
3Simply concatenate the table name, the names of the column in the index, 
4and the index type **/
5
6// Format of unique key tableName_column_unique
7$table->dropUnique('users_email_unique');

1//The UNique need be a constraint name
2// This name has this format:
3// [TABLE_NAME]_[COLUMN_NAME]_unique
4// For 'users' table and 'user_code' column, whe get the name:
5// users_user_code_unique
6$table->dropUnique('users_user_code_unique');
7
8//The inverst is 
9$table->unique('user_code');