V-data custom filter method and column filter

How can I set a column filter and still have a global filter field?
the problem: if I define a individual column filter this field is omitted by custom filter method.

Another option would be to limit the global filter on one column if user input is only one letter, but I cannot get the items key of actual value.

e.g.

   methods: {
      filterOnlyCapsText (value, search, item) {
console.log(item) // <-- returns complete item not the actual key => value
        return value != null &&
          search != null &&
          typeof value === 'string' &&
          value.toString().toLocaleUpperCase().indexOf(search) !== -1
      },
    },