How do i get the sum of values in a column of an entity in another entity?

[sujithraalink]

I have an entity called "Result". and there is a field called "score" in Result entity. each entry comes under "score" column. I need to get the sum of all entries in the "score" column and the sum has to come under another entity "Certificate" where i have a field called "grandTotal". the sum has to get in "grandTotal" field . Please help me to resolve this issue.
This is the formula i have applied in Certificate entity. there is a many to many relationship between "certificate" and "Result" entity.
grandTotal = entity\sumRelated('results', 'score');