Ruby on Rails Pitfall

Ruby on Rails PitfallOr just stupid mistakes we made

Robin LuIN-SRC Studio

robinlu@in-src.comRubyConfChina2009

Friday, May 22, 2009

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 ‘&&’

Why not this?

result = func(arg) && render(:text => result)

‘and’ or ‘&&’

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

Whentext = ‘<img title="http://example.com/x.js?" src="#"’

the page becomes:

strip_tags

strip_tags is not safe by itself

h strip_tags(text)

Controller

class Blog1Controller < ApplicationControllerdef list

unless read_fragment(:action => 'list') @articles = Article.find_recent

end end

<% cache do %> <ul> <% for article in @articles -%>

<% end %>

list.html.erb

Result:

sometime got crash due to uninitialized @articles

cachearticle list

check cache

article list

check cache list

article list

check cache list

article list

render

check cache list

article list

render

article new

check cache list

article list

expire cache

render

article new

check cache list

article list

expire cache

render

article new

check cache list

article list

expire cache

render

check cache

article new

check cache list

article list

expire cache

render

check cache

crashed by non-init @articles

article new

• defensive: handle the exception

• postpone init of @articles

• update caches instead of expiring them

Solutions?

none of them is perfect

object id

Check nil? everywhere?

object idconfig.whiny_nil = true

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_ofProcess A

Process B

unique?

Process B

unique? select ....

Process B

unique? select ....unique?

Process B

Insert

Process B

Insert

Process B

Insert

crash!

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.

conditionsWhat 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

conditionsWhen 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

create

after_create

all in one transaction

begin......commit

all the steps between this should be transactional

after_create

• send a mail

• delete a file

• expire a cache

What are non-transactional actions?

after_create

• try not put non-transaction actions into transactions.

• after_commit

• in controller

Thanks!

Ruby on Rails Pitfall

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