Relate tables with string type fields with laravel

I have some tables already related from the migration since the fields are of type int and contain a single value:

$table->unsignedInteger('user_id');
$table->foreign('user_id')->references('id')->on('users');
$table->unsignedInteger('city_id');
$table->foreign('city_id')->references('id')->on('cities');

My question is how to relate a field of type string, because in that field I keep an array of values that correspond to several id of the table to relate. Example 1,3,5,7 is the value contained in the field of my first table, the idea is to obtain the values contained in the table related to each one of those id.