Filtering Jekyll Posts by Category

One of the things that keeps catching me out when using the Liquid templating language is how to check if an array includes a specific string. Essentially I want to use the Ruby include? method but for Liquid sorta like this:

# Pseudo code - This will NOT work!
{% if post.categories.include? 'foo' %}
	Foo
{% endif %}

Answer: Use the contains operator

{% if post.categories contains 'foo' %}
	Foo
{% endif %}

For some reason I can never remember that this method is called contains and not include? so I end up rooting through the Liquid docs searching for something that doesn’t exist. And even though it’s clearly shown on the Liquid operators docs page, I never remember that it’s an operator and not some kind of where filter.

View it in the Liquid source code

You’ll find the contain method in liquid’s source code within the Condition class on the condition.rb file. It’s currently on line 20 and shows how Ruby’s include? method is used to make it work.

class Condition
	@@operators = {
		'==' => ->(cond, left, right) {  cond.send(:equal_variables, left, right) },
		'!=' => ->(cond, left, right) { !cond.send(:equal_variables, left, right) },
		'<>' => ->(cond, left, right) { !cond.send(:equal_variables, left, right) },
		'<' => :<,
		'>' => :>,
		'>=' => :>=,
		'<=' => :<=,
		'contains' => lambda do |_cond, left, right|
			if left && right && left.respond_to?(:include?)
				right = right.to_s if left.is_a?(String)
					left.include?(right)
				else
					false
				end
		end,
}

Also works for substrings

The contains Liquid method works for both arrays and strings meaning you can also do this:

# Given that the category of our post is "foobar"
{% if post.category contains 'foo' %}
	Foo
{% endif %}