Why does this happen


#1

Quick question, using ActiveRecord,

catBike = Category.new(:name => “Bikes”)
catBike.save

p = Product.new(:name => “Bike”, :price => 900)
p.categories << catBike
p.save

p.categories now has TWO catBike

however, if I move the save before adding the categories it is fine.

I am curious why this happens. Thanks!


#2

p.categories now has TWO catBike

Because << does not ask questions. It just adds a catBike. If you
already had
one, now you have two.

You might need some combination of these:

a :unique => true on the habtm (spelling?)
a unique key (index) on the database


#3

It didn’t exist before hand. ActiveRecord is triggering two inserts. I
have included a full example and the SQL

catBike = Category.new(:name => “Bikes”)
catBike.save

p = Product.new(:name => “Cannondale Bike”, :price => 1000)
p.save
p.categories << catBike # This works great.

p = Product.new(:name => “Trek Bike”, :price => 900)
p.categories << catBike # this causes two entriens to be created
p.save

Category Create (1.0ms) INSERT INTO categories (name) VALUES
(‘Bikes’)
Product Create (0.0ms) INSERT INTO products (name, price)
VALUES(‘Cannondale Bike’, 1000)
SQL (0.0ms) COMMIT
SQL (0.0ms) BEGIN
categories_products Columns (10.0ms) SHOW FIELDS FROM
categories_products
SQL (1.0ms) INSERT INTO categories_products (product_id,
category_id) VALUES (1, 1)
SQL (0.0ms) COMMIT
SQL (0.0ms) BEGIN
SQL (0.0ms) COMMIT
SQL (0.0ms) BEGIN
Product Create (1.0ms) INSERT INTO products (name, price)
VALUES(‘Trek Bike’, 900)
categories_products Columns (7.0ms) SHOW FIELDS FROM
categories_products
SQL (1.0ms) INSERT INTO categories_products (product_id,
category_id) VALUES (2, 1)
categories_products Columns (7.0ms) SHOW FIELDS FROM
categories_products
SQL (0.0ms) INSERT INTO categories_products (product_id,
category_id) VALUES (2, 1)
SQL (0.0ms) COMMIT


#4

I replicated 2 scenareos and neither of them do what you have
described:

has and belongs to many scenario

== models

class Product
has_and_belongs_to_many :categories
end

class Category
has_and_belongs_to_many :products
end

== db

create_table “categories”, :force => true do |t|
t.string “name”
end

create_table “categories_products”, :id => false, :force => true do |
t|
t.integer “product_id”
t.integer “category_id”
end

create_table “products”, :force => true do |t|
t.string “name”
t.integer “price”
end

== test

p = Product.new :name => “cool bike”, :price => 123
=> #<Product id: nil, name: “cool bike”, price: 123>

p.categories << c
=> [#<Category id: 1, name: “bikes”>]

p.save
=> true

p.categories
=> [#<Category id: 1, name: “bikes”>]

Same thing with has_many :through. which leads me to believe you
havent set your relationships right. Could you show us the mdoel and
table code plz?


#5

On 17 Mar., 15:18, ball removed_email_address@domain.invalid wrote:

Thanks. I am sure it is something I am missing. I am trying to learn
about SQL and Rails.
Here is the script I am using. I am really just trying to create a
script that explores all of the database relationships and how to
implement them in Rails.

The code:http://rafb.net/p/iUgqjX46.html

The output and SQL:http://rafb.net/p/tpvlVc44.html

I believe it’s because the << method both append the specified object
to the collection (in your case, a Category to a Products categories
collection) AND saves it afterwards. So saving the product twice might
cause the category to be added twice. At least that’s what I think. So
in your case I’ld do like this:

cat_bike = Category.new(:name => “Bikes”)
cat_bike.save

p = Product.new(:name => “Bike”, :price => 900)
p.save
p.categories << cat_bike

To avoid duplication in your categories table. Hope that was
helpful. :slight_smile:


Cheers,
David K.
http://twitter.com/rubyguy


#6

Thanks David, I poked around in the debugger a bit after your
suggestion, and honestly, I don’t understand Ruby code well enough to
make sense of it all. Not to mention, I kept running into

No sourcefile available for (eval)

However, what I did find was that the << operator, did not issue the
insert command in the “double” case. The save command after the fact
seemed to issue the two inserts. Now this confuses me completely.

So, if anyone can explain why/how this could happen, that would be
great.


#7

Thanks. I am sure it is something I am missing. I am trying to learn
about SQL and Rails.
Here is the script I am using. I am really just trying to create a
script that explores all of the database relationships and how to
implement them in Rails.

The code: http://rafb.net/p/iUgqjX46.html

The output and SQL: http://rafb.net/p/tpvlVc44.html


#8

RAFB apparently drops files after 24 hours - any chance you could put
those examples up someplace (gist or pastie, for instance) with a
little more persistence?

Without seeing the code, it’s hard to even guess what’s going on. But
I seem to recall seeing similar behavior when I had code saving an
associated record in before_save, which then got picked up again by
the association save callbacks.

–Matt J.


#9

On 18 Mar., 19:23, Matt J. removed_email_address@domain.invalid wrote:

RAFB apparently drops files after 24 hours - any chance you could put
those examples up someplace (gist or pastie, for instance) with a
little more persistence?

Without seeing the code, it’s hard to even guess what’s going on. But
I seem to recall seeing similar behavior when I had code saving an
associated record in before_save, which then got picked up again by
the association save callbacks.

I totally agree. Ball, you should really paste some code somewhere, so
we can try to figure out what’s wrong here.


Cheers,
David K.
http://twitter.com/rubyguy