Rails：调用 Model.new 时 id 字段为零

Question

I am a little confused about the auto-increment id field in rails. I have a rails project with a simple schema. When i check the development.sqlite3 I can see that all of my tables have an id field with auto increment.

CREATE TABLE "messages" ("id" INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, "text" text, "created_at" datetime, "updated_at" datetime);

But when I call Message.new in the console, the resulting object has an id of nil .

>> a = Message.new
=> #<Message id: nil, text: nil, created_at: nil, updated_at: nil>

Shouldn't the id come back populated?

Answer 1

No, that's the correct behavior. When you create an object via new (as in your example), Rails doesn't persist it to the database (just in memory).

If you do Message.create , or Message.save like theIV said, then the id will be populated.

Answer 2

Like fig said,

n = Movie.new
n.save
=> true

means that it is saved and will be given an ID. Alternatively,

n = Movie.create!

automatically saves and stores it in the database and gives it an ID with one line of code.

Answer 3

据我所知， id字段只在save s上分配，而不是在new s上分配。

Answer 4

All answers above are correct. To add to them, calling the #save method on an instance will return true if it was able to insert into database and false if it wasn't successful. Using the ::create method will make a new instance of your model, and will call #save on it.

You can also use the #save! or ::create! methods. The difference is that these will raise an error (as opposed to returning boolean) if they were not successful in their insertion to the database.

Answer 5

In my case, I called Model.create(event_id) with constant value (6) that was NOT the event's actual id. There was no error, and an Active Record Model was returned with other attributes, except id was nil.

I fixed my bug after some hours of debugging by changing the value to the actual reference id of the existing Event.id (as it was previously). Note that you might get this error if you have SQL foreign constraint to some other table, but you are using incorrect id for the reference.