es_sort_by_script.groovy

Integer priority = 0;
def sortValue = null;

for (content in _source.content) {
    // Get the priority of the source. Higher is better
    Integer thisPriority = priorities.get(content.source);
     
    // Get the value of the field we want to sort on
    def thisValue = content[sortField];
  
    // If a value exists and it has higher priority replace the chosen sort value
    if (thisValue != null && thisPriority > priority) {
        priority = thisPriority;
        sortValue = thisValue;
    }
}
 
return sortValue;