How do you delete records in a Join Table

Rails
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

Robin2 wrote:

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

Probably, you can try out by assinging empty array like

b = Boat.find[1]
b.members = []
b.save

I didn’t try out

3

What you’re doing is similar to what one would achieve using has_many
:through. It is arguably superior to HABTM since you do indeed create a
join model for your join table, so you can delete records in a typical
way. There are also other advantages to it, such as being able to store
extra data in the join table. And example would be like so:

class Post < ActiveRecord::Base
has_many :taggings, :dependent => :destroy
has_many :tags, :through => :taggings
emd

class Tag < ActiveRecord::Base
has_many :taggings, :dependent => :destroy
has_many :posts, :though => :taggings
end

class Tagging < ActiveRecord::Base
belongs_to :post
belongs_to :tag
end

So with this setup I could conceivably “tag” Posts, say in a blog type
application. This makes available methods like post.tags and
post.taggings. If I wanted to remove all the tags from a Post I could
use post.taggings.destroy_all. (It’s important not to use
post.tags.destroy_all since that would remove those Tags from every Post
that has them.) But overall you could use Tagging as you would any
other model.

I hope that helps.

4

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 …

5

On Dec 3, 2:39 pm, Robin2 [email protected] wrote:

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

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

6

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

7

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