I come across a situation where i need to implement a relational database for an application. Now my question is how to implement it and where should i save the data? also i don't want to save data on RAM as it get very expensive when data size grows.
A good way would be to use the demux-js library. It listens for specified actions and allows updating logic for whatever kind of database you like. This is a RAM free option which essentially allows you to achieve CRUD functionality by creating a mirror image of what the blockchain would show if you played through all of your actions.
Mind the mess, but you can see me implement demux here.
Another option is to use eosio mongodb_plugin for archive blockchain log into mongodb by running a node then apply queries on it.
@Nat i need your opinion about that option.