Should i use belongsTo or hasOne in Laravel?

This is not about where the foreign key is.

The OP brought up 2 scenarios: A hasOne B, A belongsTo B

For a contextual example, let A = user and B = company.

You can say "user owns(hasOne) a company" and conversely you can say "user worksAt(belongsTo) a company".

In both scenarios, user has a company_id field

Please let that sink in for a moment. Thus, discussing the location of foreign key is a non starter. The most apt answer can be found here: https://laravel.io/forum/04-20-2015-belongsto-vs-hasone

although it works, it is an incorrect description of the relationship.

To answer the OP's question: No, you should not use them interchangeably.

They are usually used in pairs

1. A belongsTo B, B hasMany A
2. B belongsTo A, A hasMany B
3. A hasOne B, B hasMany A <- this will cause problems with both sides using hasXXX, see link.

No, the difference depends on where your foreign key is.

In your example, if A has a b_id column, then A belongsTo B.

If B has an a_id column, then A hasOne or hasMany B depending on how many B should have.

Main difference is as below:

belongsTo and belongsToMany - you're telling Laravel that this table holds the foreign key that connects it to the other table.

hasOne and hasMany - you're telling Laravel that this table does not have the foreign key.