Ways to refer to a child in jQuery. I summarized it in the following jQuery

$(this).find("img"); // any img tag child or grandchild etc...   
$(this).children("img"); //any img tag child that is direct descendant 
$(this).find("img:first") //any img tag first child or first grandchild etc...
$(this).children("img:first") //the first img tag  child that is direct descendant 
$(this).children("img:nth-child(1)") //the img is first direct descendant child
$(this).next(); //the img is first direct descendant child

To enforce the constraint that each user can have only one profile in the `profiles` table, you can consider setting up a unique constraint on the `user_id` column in the table. This will ensure that each `user_id` can only appear once in the `profiles` table, thereby restricting a user to have a single profile at a time.

Here's an example of how you can achieve this using SQL in PostgreSQL