Authorize allowed values with assignable_values
All our projects have enum-like requirements like this:
- An attribute value must be included in a given set of values.
- The list of allowed values must be retrievable in order to render
<select>boxes. - Each value has a humanized label.
- Sometimes there is a default value.
Most of the time, this requirement is also needed:
- The list of assignable values depends on the user who is currently signed in.
In our past projects there are many different solutions for these related requirements, e.g. ChoiceTrait, methods like `available_…
Databases don't order rows unless you tell them so
There is no such thing as a "default order" of rows in database tables.
For instance, when you paginate a result set: When using LIMIT, it is important to use an ORDER BY clause that constrains the result rows into a unique order. Otherwise you will get an unpredictable subset of the query's rows. You might be asking for the tenth through twentieth rows, but tenth through twentieth in what ordering? The ordering is unknown, unless you specified ORDER BY.
In Rails, if you use Record.first or Record.last, it will default to orderin…
Enumerators in Ruby
Starting with Ruby 1.9, most #each methods can be called without a block, and will return an enumerator. This is what allows you to do things like
['foo', 'bar', 'baz'].each.with_index.collect { |name, index| name * index }
# -> ["", "bar", "bazbaz"]
If you write your own each method, it is useful to follow the same practice, i.e. write a method that
- calls a given block for all entries
- returns an enumerator, if no block is given
How to write a canonical each method
To write a metho…
Imagemagick: Batch resize images
Trick: Do not use convert but mogrify:
mogrify -resize 50% *
This overwrites the original image file.
In contrast, convert writes to a different image file. Here is an example if you need this:
cd /path/to/image/directory
for i in `ls -1 *jpg`; do convert -resize 50% $i "thumb_$i"; done
How to Work With Time Zones in Rails
With different time zones
When dealing with time zones in Rails, there is one key fact to keep in mind:
Rails has configurable time zones, while
Ruby is always in the server's time zone
Thus, using Ruby's time API will give you wrong results for different time zones.
For consistency, you should only use Rails' methods, but the hard thing is to know which method originates from Ruby and which from Rails. To simplify this, adhere to the following suggestion:
Use
Time.zonefor everything time-related
```
# Examp…
How to doubleclick in Selenium
Put the attached file into features/support/.
Example usage:
When /^I open the task named "(.+?)"$/ do |task_name|
task = page.all('.task').detect do |t|
t.find('.label').text.downcase == task_name.downcase
end
task.double_click
end
Note: only Selenium understands doubleclicks.\ Credits: the attached Capybara issue discussion.
Custom error pages in Rails 3.2+
Starting with Rails 3.2, there is a nicer way to display custom error pages (i.e. "page not found") that does not interfere with ExceptionNotifier. (I'm not sure about Airbrake et al.)
José Valim has posted this on his blog, here is a slightly more verbose rundown:
-
Register your own app as the application responsible for showing error pages by adding
config.exceptions_app = self.routesto your
config/application.rb. -
Add rout…
MySQL: Creating many records works faster in a transaction
When you need to insert many records into the same table, a fast way to do it is to have a single INSERT statement describing multiple rows. Unfortunately there is no way to do this through ActiveRecord.
What you can do to save time is to open a transaction and save multiple records within that transaction:
transaction do
500.times { Model.create! }
end
Although you will still trigger 500 INSERT statements, they will complete considerably faster.
When I trie…
CSS: Flex and "min-width"
min-width is known as a CSS property that can be set to define a least width for an element. Surprisingly, it can also be used to set something that feels like max-width.
min-width in a flex context
While the default min-width value is 0 (zero), for flex items it is auto. This can make block elements take up much more space than desired, even stretching their container beyond the screen edge on small screens.
[…
Shortcut for getting ids for an ActiveRecord scope
You can use .ids on an ActiveRecord 4+ scope to pluck all the ids of the relation
User.where("users.name LIKE 'Foo Bar'").ids
# same as
User.where("users.name LIKE 'Foo Bar'").pluck(:id)
MySQL: Careful when using database locks in transactions
We tend to use database transactions as a magic bullet to get rid of all our concurrency problems. When things get really bad, we might even throw in some locking mechanism, but then are usually done with it.
Unfortunately, transactions semantics in databases are actually very complicated, and chances are, your making some incorrect assumptions.
The MySQL innodb engine actually has [four different modes](ht…