private boolean isValid(String source, SummaryField summaryField, DocumentSummary summary) {
        return  isDocumentField(source) ||
                (isNotInThisSummaryClass(summary, source) && isSummaryField(source)) ||
                (isInThisSummaryClass(summary, source) && !source.equals(summaryField.getName())) ||
                (SummaryClass.DOCUMENT_ID_FIELD.equals(source));
    }
Verifies that the source fields actually refers to a valid field.

@author baldersheim
config-model/src/main/java/com/yahoo/schema/processing/SummaryFieldsMustHaveValidSource.java