I am reviewing a piece of code from a Rails project and I came across the tap method. What does it do?
Also, it would be great if someone could help me understand what the rest of the code does:
def self.properties_container_to_object properties_container
{}.tap do |obj|
obj['vid'] = properties_container['vid'] if properties_container['vid']
obj['canonical-vid'] = properties_container['canonical-vid'] if properties_container['canonical-vid']
properties_container['properties'].each_pair do |name, property_hash|
obj[name] = property_hash['value']
end
end
end
Thanks!
.tap is here to "perform operations on intermediate results within a chain of methods" (quoting ruby-doc).
In other words, object.tap allows you to manipulate object and to return it after the block:
{}.tap{ |hash| hash[:video] = 'Batmaaaaan' }
# => return the hash itself with the key/value video equal to 'Batmaaaaan'
So you can do stuff like this with .tap:
{}.tap{ |h| h[:video] = 'Batmaaan' }[:video]
# => returns "Batmaaan"
Which is equivalent to:
h = {}
h[:video] = 'Batmaaan'
return h[:video]
An even better example:
user = User.new.tap{ |u| u.generate_dependent_stuff }
# user is equal to the User's instance, not equal to the result of `u.generate_dependent_stuff`
Your code:
def self.properties_container_to_object(properties_container)
{}.tap do |obj|
obj['vid'] = properties_container['vid'] if properties_container['vid']
obj['canonical-vid'] = properties_container['canonical-vid'] if properties_container['canonical-vid']
properties_container['properties'].each_pair do |name, property_hash|
obj[name] = property_hash['value']
end
end
end
Is returning a Hash beeing filled in the .tap block
The long-version of your code would be:
def self.properties_container_to_object(properties_container)
hash = {}
hash['vid'] = properties_container['vid'] if properties_container['vid']
hash['canonical-vid'] = properties_container['canonical-vid'] if properties_container['canonical-vid']
properties_container['properties'].each_pair do |name, property_hash|
hash[name] = property_hash['value']
end
hash
end
Tap is a Ruby method from the Object class.
This method yields x to the block and then returns x. This method is used to "tap into" a method chain, to perform operations on intermediate results within the chain.
Related
I've got a class that looks like this:
class VariableStack
def initialize(document)
#document = document
end
def to_array
#document.template.stacks.each { |stack| stack_hash stack }
end
private
def stack_hash(stack)
stack_hash = {}
stack_hash['stack_name'] = stack.name
stack_hash['boxes'] = [stack.boxes.each { |box| box_hash box }]
stack_hash
end
def box_hash(box)
box_hash = {}
content = []
box.template_variables.indexed.each { |var| content << content_array(var) }
content.delete_if(&:blank?)
box_hash.store('content', content.join("\n"))
return if box_hash['content'].empty?
box_hash
end
def content_array(var)
v = #document.template_variables.where(master_id: var.id).first
return unless v
if v.text.present?
v.format_text
elsif v.photo_id.present?
v.image.uploaded_image.url
end
end
end
The document I'm testing with has two template_variables so the desired result should be a nested hash like so:
Instead I'm getting this result:
=> [#<Stack id: 1, name: "User information">]
i.e., I'm not getting the boxes key nor it's nested content. Why isn't my method looping through the box_hash and content fields?
That's because the to_array method uses each method, which returns the object it's been called on (in this case #document.template.stacks)
Change it to the map and you may get the desired result:
def to_array
#document.template.stacks.map { |stack| stack_hash stack }
end
I have a model with an initializer in it, which basically creates a user from a user hash.
After it gets the user information, it checks whether the "privileges" key in the hash is an array. If it's not, it turns it into an array.
Now the obvious way of doing this would be crafting an entire user_hash so that it would skip those "create user" lines and then check if it turns the input into an array if necessary. However, I was wondering if there is a more DRY way of doing this?
Here is the user model I'm talking about:
def initialize(opts={})
#first_name = opts[:user_hash][:first]
#last_name = opts[:user_hash][:last]
#user_name = opts[:user_hash][:user_name]
#email = opts[:user_hash][:email]
#user_id = opts[:user_hash][:id]
#privileges = {}
if opts[:privs].present?
if !opts[:privs].kind_of?(Array)
opts[:privs] = [opts[:privs]]
end
end
end
You can pass a double which returns the needed value when the proper key is requested, and itself (or something else) otherwise:
it 'turns privs into an array' do
opts = double(:opts)
allow(opts)to receive(:[]).and_return(opts)
allow(opts)to receive(:[]).with(:privs).and_return('not array')
expect(MyClass.new(opts).privileges).to eq(['not array'])
end
Btw, your code could be simplified using the splat operator:
privs = [*opts[:privs]]
sample behavior:
privs = nil
[*privs]
# => []
privs = ['my', 'array']
[*privs]
# => ["my", "array"]
privs = 'my array'
[*privs]
# => ["my array"]
You can even use the idempotent Kernel#Array
def initialize(opts = {})
#first_name = opts[:user_hash][:first]
#last_name = opts[:user_hash][:last]
#user_name = opts[:user_hash][:user_name]
#email = opts[:user_hash][:email]
#user_id = opts[:user_hash][:id]
#privileges = {}
Array(opts[:privs])
end
I hope that helps
Rather than testing the implementation (value is turned into an array), I would test the desired behavior (takes single privilege or multiple privileges):
describe User do
describe '#initialize' do
it "takes single privilege" do
user = User.new(user_hash: {}, privs: 'foo')
expect(user.privileges).to eq(['foo'])
end
it "takes multiple privileges" do
user = User.new(user_hash: {}, privs: ['foo', 'bar'])
expect(user.privileges).to eq(['foo', 'bar'])
end
end
end
How can I remove all values from ruby has. I don't want to remove keys just values.
For example:
here is my hash: {'a'=>{'b'=>'c'},'d'=>'e','f'=>{'g'=>''}}
I want this: {'a'=>{'b'=>nil},'d'=>nil,'f'=>{'g'=>nil}}
I don't want to delete the nested hashes. The nesting level varies from one to six levels
thanx
You can write custom delete_values! method, like this:
class Hash
def delete_values!
each_key do |key|
self[key].is_a?(Hash) ? self[key].delete_values! : self[key] = nil
end
end
end
{'a'=>{'b'=>'c'},'d'=>'e','f'=>{'g'=>''}}.delete_values!
# => {"a"=>{"b"=>nil}, "d"=>nil, "f"=>{"g"=>nil}}
h = {'a'=>{'b'=>'c'},'d'=>'e','f'=>{'g'=>''}}
def clean_hash h
h.each do |key, value|
if value.instance_of? Hash
clean_hash value
else
h[key] = nil
end
end
end
clean_hash h
#{"a"=>{"b"=>nil}, "d"=>nil, "f"=>{"g"=>nil}}
h = {'a'=>{'b'=>'c'},'d'=>'e','f'=>{'g'=>''}}
def cleaned_hash(h)
h.reduce({}) do |memo, (key, val)|
memo[key] = if val.is_a? Hash
cleaned_hash(val)
else
nil
end
memo
end
end
cleaned_hash h
# => {"a"=>{"b"=>nil}, "d"=>nil, "f"=>{"g"=>nil}}
This will not modify your hash but instead give you cleaned copy
I encounter a strange problem when trying to alter values from a Hash. I have the following setup:
myHash = {
company_name:"MyCompany",
street:"Mainstreet",
postcode:"1234",
city:"MyCity",
free_seats:"3"
}
def cleanup string
string.titleize
end
def format
output = Hash.new
myHash.each do |item|
item[:company_name] = cleanup(item[:company_name])
item[:street] = cleanup(item[:street])
output << item
end
end
When I execute this code I get: "TypeError: no implicit conversion of Symbol into Integer" although the output of item[:company_name] is the expected string. What am I doing wrong?
Your item variable holds Array instance (in [hash_key, hash_value] format), so it doesn't expect Symbol in [] method.
This is how you could do it using Hash#each:
def format(hash)
output = Hash.new
hash.each do |key, value|
output[key] = cleanup(value)
end
output
end
or, without this:
def format(hash)
output = hash.dup
output[:company_name] = cleanup(output[:company_name])
output[:street] = cleanup(output[:street])
output
end
This error shows up when you are treating an array or string as a Hash. In this line myHash.each do |item| you are assigning item to a two-element array [key, value], so item[:symbol] throws an error.
You probably meant this:
require 'active_support/core_ext' # for titleize
myHash = {company_name:"MyCompany", street:"Mainstreet", postcode:"1234", city:"MyCity", free_seats:"3"}
def cleanup string
string.titleize
end
def format(hash)
output = {}
output[:company_name] = cleanup(hash[:company_name])
output[:street] = cleanup(hash[:street])
output
end
format(myHash) # => {:company_name=>"My Company", :street=>"Mainstreet"}
Please read documentation on Hash#each
myHash.each{|item|..} is returning you array object for item iterative variable like the following :--
[:company_name, "MyCompany"]
[:street, "Mainstreet"]
[:postcode, "1234"]
[:city, "MyCity"]
[:free_seats, "3"]
You should do this:--
def format
output = Hash.new
myHash.each do |k, v|
output[k] = cleanup(v)
end
output
end
Ive come across this many times in my work, an easy work around that I found is to ask if the array element is a Hash by class.
if i.class == Hash
notation like i[:label] will work in this block and not throw that error
end
Suppose I have a method in a controller:
def my_find(is_published, count)
items = Idea.where(published: is_published)
#......
end
Sometimes I want to pass some extra filter arguments
def my_find(is_published, count, some_extra_filter = nil)
items = Idea.where(published: is_published) #.where (some_extra_filter)
#......
end
where some_extra_filter can be lambda or just an plain sql "where" string and it can also be nil or "".
So how do I concatenate .where(published: is_published) with where (some_extra_filter) to get what I need?
This is actually very easy using scopes:
def my_find
#items = Idea.scoped
#items = #items.where(published: is_published) unless is_published.nil?
#items = #items.where(other: other_param) if other_params < 10
# etc, etc
end