Document field path reference

The field path syntax is used several places in Vespa to traverse documents through arrays, structs, maps and sets and generate a set of values matching the expression. Examples - If the document contains the field mymap, and it has a key mykey, the expression returns the value of the map for that key: 

mymap{mykey}

Returns the value in index 3 of the myarray field, if set:

myarray[3]

Returns the value of the value1 field in the struct field mystruct, if set: 

mystruct.value1

If mystructarray is an array field containing structs, returns the values of value1 for each of those structs: 

mystructarray.value1

The following syntax can be used for the different field types, and can be combined recursively as required:

Maps/weighted Sets

<mapfield>{<keyvalue>} Retrieve the value of a specific key
<mapfield>{$<variablename>} Retrieve all values, setting the variable to the key value for each.
<mapfield>.key Retrieve all key values
<mapfield>.value Retrieve all values
<mapfield> Retrieve all keys

In the case of weighted sets, the value referenced above is the weight of the item.

Array

<arrayfield>[<index>] Retrieve the value in a specific index
<arrayfield>[$<variablename>] Retrieve all values in the array, setting the variable to the index of each.
<arrayfield> Retrieve all values in the array

Struct

<structfield>{.<subfield>} Return the value of the struct field
<structfield> Return the value of all subfields

Note that when specifying values of subscripts of maps, weighted sets and arrays, only numbers and strings may be used.

Variables

It can be useful to reference several field paths using a common variable. For instance, if you have an array of structs, you may want to use document selection on fields within the same array index together. This could be done by an expression like: 

mydoctype.mystructarray{$x}.field1=="foo" AND mydoctype.mystructarray{$x}.field2=="bar"

Variables either have a key value (for maps and weighted sets), or an index value (for arrays). Variables cannot be used across such contexts (that is, a map key cannot be used to index into an array).