@HAL I have dealt with these scenario a lot. In my experience powerbi desktop when loading data will use column names to determine if there is a relationship between two tables. For exampel you have customerID column in one table and customerID in another it will be related. However it will also look at the data so essentially determining PK FK principles.
Through lot of testing I've found a solution that has been working for me so far. Even though if you design your database properly and have all necessary PK FK relations powerbi desktop hasn't always automatically created them all. In my case I had a lot PK FK relations in database that had more than 30 tables. To avoid any manual work and manually creating relationships in powerbi desktop, I have initially used DirectQuery feature to connect to my database (I had sql server db and so DirectQuery is supported). With DirectQuery no data is imported in powerbi desktop and it maintains all PK FK relations you have in your database. Once connected save your pbix file (without doing any further work on visuals).
Next time you open that pbix file you will have button on bottom left hand corner saying DirectQuery enabled click here to change. Using that change it to import. This will load data from your tables into powerbi desktop but will maintain all PK FK relations and so you've avoided any missing out of relations that would had to be done manually.
