Active Record保存如何初始化模型？

Question

When you save a model, there are some model attributes that get their value from the database like:

1. primary key
2. column defaults set at the mysql level

When you insert a record (save), does it actually make 2 calls under the covers ie Insert and then Select ?

Answer 1

ActiveRecord doesn't make a query after the save to get that information. My understanding of ActiveRecord is that it maintains internally a lot of information about the database structure, for example the names and types of all the columns, so that it can generate the corresponding methods in the models. Also it would know what the default value is so it can set that in the model at the same time as the db sets it.

This is not completely DRY and would cause lots of problems if you bypass ActiveRecord to restructure the database, but the savings in overhead with less database touches is well worth it.