Question

“a model with a uniquely indexed column”

Does this mean just a model and a column with a unique validation on the column? Or does it mean the column needs add_index in the migration?

And could you explain what exactly it means to create an add_index. Such as if you have an Authors model, with a name column. What does adding an index to ‘name’ accomplish?

Thanks.

Answer 1

I am taking it to mean that the model has a column that is guaranteed to be unique and that there is an index on it. I take it you are reading about models in general in Rails.

A unique column means that no two models (such as User1 and User2) can have the same value for that column. For example, users would have unique logins. No two users should exist that have the same login (or username or email). But Rails automatically gives models an ID column that is always unique. Unless you change it, the first record will have ID 1, then 2, then 3, etc.

An index on a column means that it is easier to find that column. Think of a an encyclopedia. There is so much information in there, but the appendix (like an index) helps you quickly find what you are looking for. There may be an appendix of key terms, and then it will tell you where to quickly find it. That’s what an index on a column does.

So “a model with a uniquely indexed column” in Rails, by default, is the ID column: it is unique and will automatically get an index on it to more quickly find records.

Extra: when you make a model with a foreign key (example: model User may have a gender_id, and you may have a table called Gender that defines Male and Female and the gender_id corresponds to a Gender object), then you should add an index to that foreign key to make searches on it faster.

More information: http://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-add_index