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Value expression strings are used to compute a value which can
then be assigned to a given field or column. This is necessary
for both modify()
and
update()
, as well as computing values in
documents at insertion time.
An example use of a value expression string would be to
increment a counter. The expr()
function is
used to wrap strings where they would otherwise be interpreted
literally. For example, to increment a counter:
// the expression is evaluated on the server
collection.modify('true').set("counter", expr("counter + 1")).execute()
If you do not wrap the string with expr()
, it
would be assigning the literal string "counter + 1" to the
"counter" member:
// equivalent to directly assigning a string: counter = "counter + 1"
collection.modify('true').set("counter", "counter + 1").execute()