public SDField getDocumentField(String name) {
        return (SDField) documentType.getField(name);
    }
Returns a field defined in one of the documents of this schema.
This does not include the extra fields defined outside the document
(those accessible through the getExtraField() method).

@param name the name of the field to return
@return the named field, or null if not found
config-model/src/main/java/com/yahoo/schema/Schema.java