Consider following tables and java classes to phrase the problem for which I seeking solution –
CITY table having columns id, name. Java class City having id and name property.
COUNTRY table having columns id, name. Java class Country having id and name property.
USER table having columns id, name, city_id, country_id where city_id is FK from CITY table and country_id is FK from COUNTRY table. Java class User having id, name, cityId and countryId property.
Now on the UI one cannot show cityId and countryId for the user record instead names to be shown as Ids are meaningless for users. So is my User class design correct? What should be the design? Should it be City and Country objects instead of cityId and countryId? Also how should I load the required data?
Imagine the same for a complex system where the tables has more columns. How would the design scale for a complex system? As I do not need the whole objects all the time only name is enough most of the time.
first, your design doesn’t sound right. a city should belong to a country and a user should not need a direct reference to a country since the user’s country is accessible through their city.
but to address the memory use: you have various options. often, if you look at the numbers, the total number of cities and countries is not so great, so you can simply use direct links to objects. i have applications where these kind of “related info” classes are kept in memory continuously – that reduces the amount of churn when reading and writing the important data (like people).
alternatively, you can extend the user class with a separate (read only) location attribute (not modifiable or written to disk) which is populated on read by joining the city and country names as a string. then you read the people, populating that field, but not populating the city and country objects (or by making them lazy). that gives you data to display without loading all the objects into memory (but you still have the memory cost of the strings).
or if you are really pushed for space you make the retrieval of location be part of the display itself and only retrieve and display locations for the people that are visible in the display. but this is a lot of work to do well.