How to store select name="field[index] in jsonb in Rails?

Question

Ruby on Rails

Greetings, I can’t figure out how to do it.

There is a pack of elements

<select class="form-control " name="item[fields_mapping][1]" id="item_fields_mapping">
options....
</select>
<code lang="html">
<select class="form-control "  name="item[fields_mapping][2]" id="item_fields_mapping">
options....
</select>
<code lang="html">
<select class="form-control "  name="item[fields_mapping][3]" id="item_fields_mapping">
options....
</select>
</code>


база данных postgres, поле fields_mapping тип jsonb
не сохраняется json объект, сохраняется только значение первого элемента select