Ruby: Referencing global variables with the built-in English library
With Ruby's build-in library English you can reference global variables with an english name. This makes you code easier to read and is also suggested by Rubocop's Style/GlobalVars cop.
Example before:
if 'foo' =~ /foo/
puts $~[1] # => foo
end
Example after:
if 'foo' =~ /foo/
puts $LAST_MATCH_INFO[1] # => foo
end
Require pitfall in Rails
The English library is not loaded by default in Rails. S...
How Rails and MySQL are handling time zones
When working with times and dates in Rails applications, you need to deal with the following problem:
- In Rails,
Timeobjects have a time zone. You can get the zone name by doingtime_object.zone. - This zone is considered when doing time calculations, e.g. 10 AM CEST minus 8 AM UTC is zero.
- A datetime in MySQL does not have a zone. It just stores the literal string "2010-05-01 12:00:00".
- That means that Rails must make assumptions about timestamps loaded from and written to MySQL.
Rails has two completely different modes ...
PostgreSQL: WITH Queries (Common Table Expressions)
PostgreSQL's Common Table Expressions (CTEs) can be used to extract sub-queries from bulky SQL statements into a temporary table to be referenced instead.
This is most useful to avoid ugly joins or sub-selects. CTEs can be used for SELECT, INSERT, UPDATE or DELETE.
Example (from the PostgreSQL docs):
WITH regional_sales AS (
SELECT region, SUM(amount) AS total_sales
FROM orders
GROUP BY region
), top_regions AS (
SELECT region
FROM regional_sales
WHERE total_sales > (SELEC...
count vs. size on ActiveRecord associations
TLDR
- When counting records in an association, you should use
#sizein most cases. - It will not work if the parent record has never been saved. Also there are finer distinctions between
#sizeand#count. See below.
count
- Always makes a
COUNT(*)query if a counter cache is not set up. - If a counter cache is set up on the association,
#countwill return that cached value instead of executing a new query.
size, if the association has already been loaded
- Counts elements in the already loaded array.
- Does not ...
BigDecimal arithmetic in Ruby
Ruby comes with a class BigDecimal which you can use for arbitrary precision arithmetic. You should use BigDecimal instead of Float whenever you care about rounding errors, e.g. whenever you are dealing with money.
You should remember these two rules when working with BigDecimal values:
- When you add or multiply a
BigDecimalwith anotherBigDecimal, the ...
Lazy-loading images
Since images are magnitudes larger in file size than text (HTML, CSS, Javascript) is, loading the images of a large web page takes a significant amount of the total load time. When your internet connection is good, this is usually not an issue. However, users with limited bandwidth (i.e. on mobile) need to mine their data budget better.
One popular strategy to improve the website performance is to not load images until they enter the viewport – aka "lazy-loading images".
General Issues
- Crawlers do not execute JavaScript (generally sp...
How to evaluate CSS media queries in JavaScript
To make CSS rules dependent on the screen size, we use media queries:
@media (max-width: 500px) {
// rules for screen widths of 500px or smaller
}
Browsers will automatically enable and disable the conditional rules as the screen width changes.
To detect responsive breakpoints from JavaScript, you may use the global matchMedia() function. It is supported in all brow...
How to grep through the DOM using the Capybara API
When your Cucumber feature needs to browse the page HTML, and you are not sure how to express your query as a clever CSS or XPath expression, there is another way: You can use all and find to grep through the DOM and then perform your search in plain Ruby.
Here is an example for this technique:
Then /^I should see an image with the filename...
Deterministic ordering of records by created_at timestamp
Creating records in specs can be so fast that two records created instantly after one another might have the same created_at timestamp (especially since those timestamps don't have an indefinitely high resolution). When ordering lists by timestamps, you should therefore always include a final order condition using the primary key of the table.
class Photo < ActiveRecord::Base
scope :by_date, -> { order('created_at DESC, id DESC') }
end
Photo.by_date
Remember to include the id field in the database index.
PSA: "index: true" in Rails migrations does not work as you'd expect
Several Rails migration methods accept index: true as an option to create an index. In some cases (like #add_column), this option is silently discarded. Know what you are doing, or use #add_index instead.
Example
Consider the following migration.
class CreateExamples < ActiveRecord::Migration
def change
create_table :examples do |t|
t.references :category, index: true
t.boolean :positive, index: true
t.integer :number_of_participants, index: true
end
add_referen...
Ruby: A small summary of what return, break and next means for blocks
Summary
- Use
returnto return from a method.returnaccepts a value that will be the return value of the method call. - Use
breakto quit from a block and from the method that yielded to the block.breakaccepts a value that supplies the result of the expression it is “breaking” out of. - Use
nextto skip the rest of the current iteration.nextaccepts an argument that will be the result of that block iteration.
The following method will serve as an example in the details below:
def example
puts yield
puts 'don...
Active Record: Named bindings in conditions
In Active Record you can use named bindings in where-conditions. This helps you to make your code more readable and reduces repetitions in the binding list.
Example without named bindings
User.where(
'name = ? OR email ?',
params[:query],
params[:query]
)
Example with named bindings
User.where(
'name = :query OR email :query',
query: params[:query]
)
Async control flow in JavaScript: Promises, Microtasks, async/await
Slides for Henning's talk on Sep 21st 2017.
Understanding sync vs. async control flow
Talking to synchronous (or "blocking") API
print('script start')
html = get('/foo')
print(html)
print('script end')
Script outputs 'script start', (long delay), '<html>...</html>', 'script end'.
Talking to asynchronous (or "evented") API
print('script start')
get('foo', done: function(html) {
print(html)
})
print('script end')
Script outputs 'script start', `'...
How to use Parallel to speed up building the same html partial multiple times (for different data)
The parallel-gem is quite easy to use and can speed up rendering time if you want to render the same partial multiple times (e.g. for rendering long lists of things).
If your parallelized code talks to the database, you should ensure not to leak database connections.
Consider you want to render a list of groups with their members as json. You can use a partial for the rendering of group members, b...