How do you delete records in a Join Table

rubyonrails-talk
1

What is the correct way to delete the records in a Join Table (used to link two files using habtm).

I have resorted to creating a model for the join table. The table is called boats_members (as it links two tables called boats and members. And the model is called boats_member (to allow for rails pluralizing). This way I can do Boats_member.delete_all which works just fine. (I am also deleting all the records from the boats and members file prior to rebuilding the whole thing). Because you do not normally need a model for a join table I suspect there is a "more correct" way to delete the records.

I have tried using .clear but that did not delete the records in the join table. I think what I tried was     b = Boat.find[1]     b.members.clear but I have deleted the non-working code and I can't be sure. I'm pretty sure I tried the .clear after had deleted the records in the boats and members tables, in case that matters.

I am concerned that if junk remains in the join table it may accidentally represent an incorrect join between unrelated records - as well as which the join table might grow to an enormous size if it cannot be cleared out.

Thanks

2

When you destroy an active record, RAILS automatically deletes any corresponding entries in join tables.

class Bee < ActiveRecord:Base   has_and_belongs_to_many :seas end

b = Bee.find(...)

b.destroy # Deletes all rows in beas_seas with bee_id = b.id

3

Thanks,

It looks like my problem was using delete_all when I should have used destroy_all. The distinction is not clear from the Rails API - surprise, surprise ...

4

Be advised that bulk destroy_all can be very slow in some cases. Using
a model and through associations and delete_all can be 5x faster or
more.

Michael

Michael

5

Thanks,

speed is irrelevant - I only want to use this a few times while I am developing the app - I am transferring data from an Access database and splitting it into relational tables. I need to be able to update the the latest data when I get the transfer process working properly (which it now seems to be) and when I am given the latest data (tomorrow?)

As an aside, you wouldn't believe the mess the Access database is in - all in a single table with, for example separate columns for first child (i.e. young person), second child etc. And I have cobbled together a rails app to replace most of it in a couple of days. It would have taken me longer to re-learn how to use Access as I have only used it a little bit.

This is an app for my own use as I take over the operation of a club membership system - so no pressure to make it look nice :slight_smile:

...Robin