如果我将活动记录与ruby一起使用，是否真的需要创建中间SQL表？

Question

I read a book and I from what I've seen I can't even think of one case that justifies using an intermediate SQL table to relate ID's as I was thought in University a good database design should have...the model specifies that kind of relations. I come fdrom the raw SQL world and now that I've discovered active record I don't see the need for relating id's on tables anymore. Am I wrong on this one?. If I am, please, enlighten me on why should I keep creating those intermediate SQL tables...

Answer 1

Intermediate tables -- such as those with only foreign keys in them -- are necessary for many-to-many associations. For instance if you have a relationship between students and classes in which a student can belong to multiple classes and classes can have multiple students, then you'll need such a join table.

This is inherent in the relational database architecture, and is not changed by adding ActiveRecord to the mix. ActiveRecord can make it easy to ignore such intermediate tables when writing queries, but it needs to include them when building queries.

Consider how you would associate many students to classes that in turn have many students, without a join table. Either each student record would have to have multiple columns containing foreign keys to different classes, or multiple student records with the same data, but different foreign keys of a class would be needed. The first method would limit the number of associations to the number of foreign key columns. The second method would break the normalization of the database by storing the same data in multiple records.

By creating a join table instead, a student <-> class association can be recorded by simply adding a record to the join table. Any number of these associations can be created (up to the db table size limitation), and no data need be repeated.