Throw error in Mongoid - ruby-on-rails

Question

Lets say i have this code:
map = %Q{
function() {
emit(this.name, { likes: this.likes });
}
}
reduce = %Q{
function(key, values) {
var result = { likes: 0 };
values.forEach(function(value) {
if(value.likes < 0){
#{Rails.logger.error "likes are negativ" }
}
result.likes += value.likes;
});
return result;
}
}
Band.where(:likes.gt => 100).map_reduce(map, reduce).out(inline: true)
As you can see I want to record an error if the value.likes are negativ:
#{Rails.logger.error "likes are negativ" }
But this Code is executed each time I run the aggregate and not when the likes are negativ.
What can I do to throw an error in the aggregate statement?

Answer 1

Lets just analyze the code. Firstly:
map = %Q{
function() {
emit(this.name, { likes: this.likes });
}
}
Here a string is assigned to a variable map. Please not the %Q{} is just another way of writing "". Former is another syntax to easily define strings which have a double quote. e.g.
# pretty
%Q{He said "You are awesome"}
# not so pretty
"He said \"You are awesome\""
Next there is:
reduce = %Q{
function(key, values) {
var result = { likes: 0 };
values.forEach(function(value) {
if(value.likes < 0){
#{Rails.logger.error "likes are negative" }
}
result.likes += value.likes;
});
return result;
}
}
Here another string is assigned to a variable reduce. #{Rails.logger.error "likes are negative" } is just a regular string interpolation logging an error and returning true. So above code is equivalent to:
Rails.logger.error "likes are negative"
reduce = %Q{
function(key, values) {
var result = { likes: 0 };
values.forEach(function(value) {
if(value.likes < 0){
true
}
result.likes += value.likes;
});
return result;
}
}
You see why the logging statement is executed every time.
Next there is:
Band.where(:likes.gt => 100).map_reduce(map, reduce).out(inline: true)
This is just a simple statement using mongoid to execute a map-reduce command on the mongo server, passing map and reduce functions constructed earlier.
Note that in above code, intention is to execute ruby code in a javascript reduce function. However that is not possible, as reduce function is being executed on mongodb server and cannot execute the logging statement.
One way to handle the situation could be to reduce to a hash like {likes: 0, negative_likes: 0}, incrementing negative_likes conditionally and logging error on receiving result.
PS: it might be a better idea to use aggregation framework instead of map-reduce.