Running Rspec examples by name, or: Running a single shared example
When an Rspec example fails, I usually investigate by running that example again using rspec <file:line>. However, this does not work with shared examples, since Rspec doesn't know in which context the shared example should be run.
But there is a different way: You can run the shared example using the -e, --example option. It takes a string value and runs all scenarios containing that substring in their full description.
This allows you to run a single uniquely named example, all examples with
similar names, all the examples in a u…
Rails and Postgres: How to test if your index is used as expected
This is a small example on how you can check if your Postgres index can be used by a specific query in you Rails application. For more complex execution plans it might still be a good idea to use the same path of proof.
1. Identify the query your application produces
puts User.order(:last_name, :created_at).to_sql
# => SELECT "users".* FROM "users" ORDER BY "users"."last_name" ASC, "users"."created_at" ASC
2. Add an index in your migration and migrate
add_index :users, [:last_name, :created_at]
3. Test the…
Differences between transactions and locking
Web applications can be used by multiple users at the same time. A typical application server like Passenger has multiple worker processes for a single app. In a distributed deployment setup like we use at makandra you will even have multiple application servers, each with their own worker pool.
This means that your code needs to deal with concurrent data access. The two main tools we use to cope with concurrency are database transactions and distributed locks. These two are not interchangeable. You ca…
makandra-rubocop 4.0.0 released
From time to time we update the underlying rubocop version in our makandra-rubocop gem. This often results in breaking changes in the cops configuration.
The upgrade process is described in the Changelog. Normally you will only see the new cops that were added by the rubocop upstream. They will result in new offenses in your project and need to be fixed.
For this release we want to encourage you to enable the Rails cops, too. This is how you co…
Rails: Overwriting default accessors
All columns of a model's database table are automagically available through accessors on the Active Record object.
When you need to specialize this behavior, you may override the default accessors (using the same name as the attribute) and simply call the original implementation with a modified value. Example:
class Poet < ApplicationRecord
def name=(value)
super(value.strip)
end
end
Note that you can also avoid the original setter and directly read/write from/to the instance's attribute storage. However th…
Configuring RubyGems to not install documentation by default
When installing gems, a lot of time is spent building locally installed documentation that you probably never use.
We recommend you disable documentation generation for gem install by default.
Note that Bundler won't install documentation, so this advice applies only when installing gems manually.
If you don't already have it, create a ~/.gemrc file. The gemrc is a Yaml file, so add the following line to add default switches to the gem command.
gem: --no-document
(If you do n…
XHR is not JSON
When a Rails controller action should handle both HTML and JSON responses, do not use request.xhr? to decide that. Use respond_to.
I've too often seen code like this:
def show
# ...
if request.xhr?
render json: @user.as_json
else
# renders default HTML view
end
end
This is just plain wrong. Web browsers often fetch JSON via XHR, but they (should) also send the correct Accept HTTP header to tell the server the data they expect to receive.
If you say request.xhr? as a means for "…
Updated: How to make changes to a Ruby gem (as a Rails developer)
Updated partial:
Tests
- Many gems have continuous integration tests run on Travis CI. You need to sign in to Travis to get build notifications. Note that you should only authorize Travis for Open Source and not Travis Pro to not violate our recommended permission guideline for Github accounts.
Linux Performance Analysis in 60,000 Milliseconds
You login to a Linux server with a performance issue: what do you check in the first minute?
uptime
dmesg | tail
vmstat 1
mpstat -P ALL 1
pidstat 1
iostat -xz 1
free -m
sar -n DEV 1
sar -n TCP,ETCP 1
top
MySQL will not use indexes if you query the wrong data type
When MySQL refuses to use your index, you can do a number of things wrong. One of them might be conditions with improper data types.
An example
For example, let's assume you have a users table with an email field (varchar) which is indexed.
MySQL will use the index when your query is well-formed:
mysql> EXPLAIN SELECT * FROM users WHERE email = 'foo@example.com';
+----+-------------+-------+-------+----------------------+----------------------+---------+-------+------+-------+
| id | select_type | table | type | poss...
Updated: JavaScript bookmarklet to click an element and copy its text contents
- Now highlights element underneath mouse pointer
- Cleans up blank lines in element text
Defining custom elements in HTML
A while ago I tweeted a thread about how a small JavaScript snippet, one that can fit in a single tweet in fact, can be used to allow defining custom elements purely in HTML. This post will expand on the idea, show how the snippet works, and argue for why you might want to actually use this.
A nice trick that lets you define "partials" in HTML without any additional rendering technology on the server or client.
Pattern: Disabling a certain feature in tests
There is a kind of features in web applications that hinder automated integration tests. Examples include cookie consent banners or form captchas. Clearly, these should be disabled so you do not have to explicitly deal with them in each and every test (like, every test starting with accepting the cookies notice). On the other hand, they must be tested as well.
A good feature disabling solution should therefore meet these requirements:
-
The feature is generally disabled in tests. A test does not need to do anything manually.
-
It is *…