Specifies fields to be updated as an expression.
Expression fields are cases such as counter = counter + 1. This method takes precedence over MergeQuery::updateFields() and it's wrappers, MergeQuery::key() and MergeQuery::fields().
$field: The field to set.
$expression: The field will be set to the value of this expression. This parameter may include named placeholders.
$arguments: If specified, this is an array of key/value pairs for named placeholders corresponding to the expression.
MergeQuery The called object.
public function expression($field, $expression, array $arguments = NULL) {
$this->expressionFields[$field] = array(
'expression' => $expression,
'arguments' => $arguments,
);
$this->needsUpdate = TRUE;
return $this;
}