Ruby on Rails Pitfall
Or just stupid mistakes we made
Robin Lu
IN-SRC Studio
robinlu@in-src.com
RubyConfChina2009

IN-SRC Studio
• http://www.in-src.com
• Team behind Caibangzi.com
• Full stack Ruby On Rails Development
• Projects from Pepboys,Vitality, Healthwise...

‘and’ or ‘&&’
What does this mean?
result = func(arg) and render(:text => result)

‘and’ or ‘&&’
What does this mean?
result = func(arg) and render(:text => result)
Why not this?
result = func(arg) && render(:text => result)

‘and’ or ‘&&’
What does this mean?
result = func(arg) and render(:text => result)
Why not this?
result = func(arg) && render(:text => result)
Be aware of the operator precedence

strip_tags
Display user input text without tags
What we did:

strip_tags
When
text =
‘<img title=\"http://example.com/x.js?\" src=\"#\"’
the page becomes:
<p> <img title=\"http://example.com/x.js?\" src=\"#\" </p>

strip_tags
strip_tags is not safe by itself
h strip_tags(text)

cache
class Blog1Controller < ApplicationController
def list
unless read_fragment(:action => 'list')
@articles = Article.find_recent Controller
end
end
end
<% cache do %>
<ul>
<% for article in @articles -%>
<li><p><%= h(article.body) %></p></li> list.html.erb
<% end -%>
</ul>
<% end %>

cache
Result:
sometime got crash due to
uninitialized @articles

cache
article list

cache
article list
check cache

cache
article list
check cache list

cache
article list
check cache list
render

cache
article list article new
check cache list
render

cache
article list article new
check cache list expire cache
render

cache
article list article new
check cache list expire cache
render

cache
article list article new
check cache list expire cache
render
check cache

cache
article list article new
check cache list expire cache
render
check cache
crashed by non-init
@articles

cache
Solutions?
• defensive: handle the exception
• postpone init of @articles
• update caches instead of expiring them
none of them is perfect

whiny nil

whiny nil
Check nil? everywhere?

whiny nil
config.whiny_nil = true

validate_uniqueness_of

validate_uniqueness_of
We always get errors like this:
A ActiveRecord::StatementInvalid occurred in
fund#add_watch_fund:
Mysql::Error: Duplicate entry '1234-271' for key 2:
INSERT INTO `watch_funds` (`account_id`,
`position`, `fund_id`, `created_at`) VALUES(1234, 19,
271, '2009-05-06 19:13:50')

validate_uniqueness_of
Process A
Process B

validate_uniqueness_of
Process A
Process B
unique?

validate_uniqueness_of
Process A
Process B
unique? select ....

validate_uniqueness_of
Process A
Process B
unique? select ....
unique?

validate_uniqueness_of
Process A
Process B
unique? select ....
unique?
Insert

validate_uniqueness_of
Process A
Process B
unique? select ....
unique?
Insert
Insert

validate_uniqueness_of
Process A
Process B
unique? select ....
unique?
Insert
Insert
crash!

validate_uniqueness_of
validate_uniqueness_of may not
guarantee the uniqueness
use your own lock if the uniqueness is
critical to you.

conditions
Background:
• category has many subcategories
• subcategory has many posts
• post belongs to subcategory
we need to select all posts in a category.

conditions
What we did:
named_scope :in_category, lambda { |cat|
conditions = [cat.subcategories.map {|subcat|
'posts.subcategory_id = ?'
}.join(\" OR \")]
cat.subcategories.each {|subcat|
conditions << subcat.id }
{:conditions => conditions}
}

conditions
Result:
we get all posts when a category has no
subcategories

conditions
When category has no subcategory
named_scope :in_category, lambda { |cat|
conditions = [cat.subcategories.map {|subcat|
'posts.subcategory_id = ?'
}.join(\" OR \")]
cat.subcategories.each {|subcat|
conditions << subcat.id }
{:conditions => conditions}
}

conditions
When you compose conditions, be
aware that sometime nothing to
compose means
the conditions should match nothing,
not the conditions should be empty.

before_create
set a flag if the author of the post is an admin
What we did:

before_create
Result:
Only post by admin can be saved

before_create
All these callbacks are Filters
Be careful not to break the filter chain by
what you return from the filters!

after_create
send a mail whenever a new record is created
What we did:

after_create
Result:
sometime the record save failed but we
still get mail notification

after_create
before_create begin
...
create ...
commit
after_create
all in one transaction all the steps between this
should be transactional

after_create
What are non-transactional actions?
• send a mail
• delete a file
• expire a cache

after_create
• try not put non-transaction actions into
transactions.
• after_commit
• in controller

Thanks!