In partial view, we can use the instance variable directly, but it may be confused and make it hard to reuse anywhere because we don’t know exactly which instance variable can be used, so use the local variable in partial with the explicitly assignment. Always replace instance variable with local variable.
A partial is a reusable view template; it allows you to modularize the components which make up a particular page into logical, cohesive pieces. When required data is not passed into a partial, it is often difficult to reuse or change later. By passing the necessary data in as locals, you create self-documenting requirements for each view partial. It also helps to see where else it is rendered and what locals are required.
In this example, the partial sidebar can use the instance variable @post, but we can’t know what’s the instance variable @post without checking the controller codes. Let’s use the local variable instead.
Now we can use the local variable post in partial, it is much simpler.