I know it's a very short question. I understand "{ }" represents a loop.
and the new operator creates a new active record object.
What does this line do in rails? from where does lead come?
Proc.new{|lead| lead.lead_details.name}
It creates new Proc object. lead doest't come from anywhere in this example since this Proc doesn't get called. But you can call that, passing it as a block, for example.
leads = Lead.includes(:lead_details) # I assume it's an AR model, obviously
p = Proc.new { |lead| lead.lead_details.name }
names = leads.map(&p)
This way, lead comes from map method and represent single element of leads array-like object, it's equivalent to this:
leads.map { |lead| lead.lead_details.name }
You can also call this procedure 'by hand', passing argument explicitly, like this:
p.call(leads.first)
# => Whatever is leads.first.lead_details.name
You can even write your own method using it as block, for example:
def first_do(collection)
yield(collection.first)
end
first_do(leads, &p)
# => Whatever is leads.first.lead_details.name
Related
i'm trying (and actually succeded, but i don't understand how it works) to write a custom method for a hash in my model (I'm working on Ruby on Rails 6).
My hash looks like this
my_hash = {
[['name_1', 'slug_1']=>value_1],
[['name_2', 'slug_2']=>value_2],
[['name_1', 'slug_1']=>value_3],
[['name_2', 'slug_2']=>value_4]
}
So basically a hash of arrays. You notice that the 'keys' are arrays that repeat themselves many times, but with different values. What i want to achieve is to write a custom method that "joins" all the keys in only one key, which will have an array of values assigned, so basically i should be able to get:
my_hash = {
['name_1', 'slug_1']=>"values": [value_1, value_3],
['name_2', 'slug_2']=>"values": [value_2, value_4]
}
For that, I have this piece of code, which i use many times:
my_hash.inject({}) do |hash, record|
# each record has the following format => [["unit_name", "axis.slug"]=>average_value(float)]
keys, value = record
# now keys has ["unit_name", "axis.slug"] and values equals average_value
hash[keys.first] ||= {}
hash[keys.first][keys.last] = value.to_f
hash
end
Since I use this many times, i wanted to write a custom method, so i did:
def format_hash_data my_hash
my_hash.inject({}) do |hash, record|
# each record has the following format => [["unit_name", "axis.slug"]=>average_value(float)]
keys, value = record
# now keys has ["unit_name", "axis.slug"] and values equals average_value
hash[keys.first] ||= {}
hash[keys.first][keys.last] = value.to_f
hash
end
end
And used it like: my_hash = format_hash_data(my_hash) with no success(it threw an error saying that 'format_hash_data' was not a valid method for the class).
So I fiddled around and added 'self' to the name of the method, leaving:
def self.format_hash_data my_hash
my_hash.inject({}) do |hash, record|
# each record has the following format => [["unit_name", "axis.slug"]=>average_value(float)]
keys, value = record
# now keys has ["unit_name", "axis.slug"] and values equals average_value
hash[keys.first] ||= {}
hash[keys.first][keys.last] = value.to_f
hash
end
end
Which, to my surprise, worked flawlessly when using my_hash = format_hash_data(my_hash)
I don't really understand why adding 'self' makes my code works, maybe anyone can shed some light? I tried using things like send() or instance_eval first, to just send the piece of code to the actual hash as a method (something like my_hash.instance_eval(my_method)) but I couldn't get it working.
I'm sorry about the long explanation, I hope i was clear enough so any of you who had this same dilemma can understand. Thanks in advance.
Prepending self. to the method name makes it a class method instead of an instance method. If you are not sure of the difference, you should look it up as it is fundamental to properly defining and using classes and methods.
As a class method, you would use it as:
my_hash = MyHash.format_hash_data(my_hash)
Or if you're in scope of the class, simply my_hash = format_hash_data(my_hash), which is why it worked in your case with the self. prepended (class method definition).
If you want to define it as an instance method (a method that is defined for the instance), you would use it like so:
my_hash = my_hash.format_hash_data
And the definition would use the implicit self of the instance:
def format_hash_data
self.inject({}) do |hash, record|
# each record has the following format => [["unit_name", "axis.slug"]=>average_value(float)]
keys, value = record
# now keys has ["unit_name", "axis.slug"] and values equals average_value
hash[keys.first] ||= {}
hash[keys.first][keys.last] = value.to_f
hash
end
end
I have a Model user with the following method:
def number_with_hyphen
number&.insert(8, "-")
end
When I run it several times in my tests I get the following output:
users(:default).number_with_hyphen
"340909-1234"
(byebug) users(:default).number_with_hyphen
"340909--1234"
(byebug) users(:default).number_with_hyphen
"340909---1234"
(byebug) users(:default).number_with_hyphen
"340909----1234"
It changes the number ?Here are the docs https://apidock.com/ruby/v1_9_3_392/String/insert
When I restructure my method to:
def number_with_hyphen
"#{number}".insert(8, "-") if number
end
If works like expected. The output stays the same!
How would you structure the code, how would you perform the insert?
which method should I use instead. Thanks
If you're using the insert method, which in the documentation explicitly states "modifies str", then you will need to avoid doing this twice, rendering it idempotent, or use another method that doesn't mangle data.
One way is a simple regular expression to extract the components you're interested in, ignoring any dash already present:
def number_with_hyphen
if (m = number.match(/\A(\d{8})\-?(\d+)\z/))
[ m[1], m[2] ].join('-')
else
number
end
end
That ends up being really safe. If modified to accept an argument, you can test this:
number = '123456781234'
number_with_hyphen(number)
# => "12345678-1234"
number
# => "123456781234"
number_with_hyphen(number_with_hyphen(number))
# => "12345678-1234"
number_with_hyphen('1234')
# => "1234"
Calling it twice doesn't mangle anything, and any non-conforming data is sent through as-is.
Do a clone of the string:
"#{number}".clone.insert(8, '-')
The model User has first, last and login as attributes. It also has a method called name that joins first and last.
What I want is to iterate through the Users records and create an array of hashes with the attributes I want. Like so:
results = []
User.all.map do |user|
record = {}
record["login"] = user.login
record["name"] = user.name
results << record
end
Is there a cleaner way in Ruby to do this?
Trying to map over User.all is going to cause performance issues (later, if not now). To avoid instantiating all User objects, you can use pluck to get the data directly out of the DB and then map it.
results = User.all.pluck(:login, :first, :last).map do |login, first, last|
{ 'login' => login, 'name' => first << last }
end
Instantiating all the users is going to be problematic. Even the as_json relation method is going to do that. It may even be a problem using this method, depending on how many users there are.
Also, this assumes that User#name really just does first + last. If it's different, you can change the logic in the block.
You can use ActiveRecord::QueryMethods#select and ActiveRecord::Relation#as_json:
User.select(:login, '(first || last) as name').as_json(except: :id)
I would write:
results = User.all.map { |u| { login: u.login, name: u.name } }
The poorly named and poorly documented method ActiveRecord::Result#to_hash does what you want, I think.
User.select(:login, :name).to_hash
Poorly named because it does in fact return an array of Hash, which seems pretty poor form for a method named to_hash.
I am using a recursive function.
def abc
get_course_to_be_deleted( courses, array_course_name, nil )
end
def self.get_course_to_be_deleted( courses, array_course_name, course )
if array_course_name.blank?
#### array_course_name = []
course
else
#### array_course_name = ['Science', 'Maths']
array_course_name.each do |course_name|
course = courses.detect{ |course| course.name == course_name }
course_names = array_course_name - [ course_name ]
get_course_to_be_deleted( course.children, course_names, course )
end
end
end
Tried .empty? its not working! array_course_name is always an array, assume a case i have three courses in array_course_names say [ 'Science', 'Botany', 'Zoology']. For the first time the course object will be Science object, course_names would be ['Botany', 'Zoology'], course.children would be botany object. as same it continues to execute the loop.At the last cycle array_course_names would be blank, course would be Zoology object, in that case i would like to return the found zoology object to calling function, but it is NOT getting returned to calling function instead it goes to else block with array_course_names as ['Botany', 'Zoology'] and which throws an error 'undefined method children for nil class' since there is no course exists. How to exit from recursive function when a condition is satisfied??
In your last line, where you perform the recursion, you are submitting an Array (formed by the line course_names = array_course_name - [ course_name ]) rather than a String. However, in your test of if array_course_name.blank?, you test as though you passed a String.
Either pass a String on the recursion or change your test to see if the Array is empty (or some other similar base case that meets your needs) instead of checking if a String is blank?.
This next bit is beyond the likely scope of your OP, but just in case it's an interest of yours: if you want to support both Arrays and Strings as the type of second parameter, you'll have to add .class/.kind_of? support for that.
I want to define a class method that has access to a local variable. So this would be different for each instance of the class. I know you can make a class method dynamic with lambda like when you use it with named_scope. But can this be done for values that are specific to an instance?
In detail it is the has_attached_file method for the paperclip plugin in rails. I want to pass a lambda for the styles hash so that the image styles can be based off of attributes of the object stored in the DB. Is this possible?
Disclaimer: First, the question (Can you pass self to lambda?) and the problem you're trying to solve (dynamic styles with paperclip) don't fully match up. I won't answer the original question because it's not entirely related to your problem, and rampion took a valiant stab at it.
I'll instead answer your paperclip question.
In detail it is the has_attached_file method for the paperclip plugin in rails. I want to pass a lambda for the styles hash so that the image styles can be based off of attributes of the object stored in the DB. Is this possible?
Yes, it is possible. In paperclip, the :styles option can take a Proc. When the attachment is initialized, if a Proc was used, the attachment itself is passed to the Proc. The attachment has a reference to the associated ActiveRecord object, so you can use that to determine your dynamic styles.
For example, your has_attached_file declaration might look something like this (assuming a User and avatar scenario where the user can customize the size of their avatar):
class User < ActiveRecord::Base
has_attached_file :avatar, :styles => lambda { |attachment|
user = attachment.instance
dimensions = "#{user.avatar_width}x#{user.avatar_height}#"
{ :custom => dimensions }
}
end
Ok, you're being unclear.
Local variables in ruby begin with a lowercase letter (like foo, bar, or steve), and are lexically scoped (like C variables). They have nothing to do with "an instance of a class"
Instance variables in ruby begin with an # sigil (like #foo, #bar, or #carl), and are in scope whenever the current value of self is the object they are stored in.
If you want a method that can access the instance variables of an object directly, that's called an instance method. For example, battle_cry and initialize are both instance methods:
class Character
def initialize(name)
#name=name
end
def battle_cry
#name.upcase + "!!!"
end
def Character.default
new("Leeroy Jenkins")
end
end
A class method, by contrast, is a method for a Class object, and doesn't have access to any of the instance variables of that object. In the above example,
default is a class method.
If you want a (class or instance) method that triggers a change in or gets a value from the current scope, ruby uses a type of callback called a block.
class Character
ATTACKS = [ "Ho!", "Haha!", "Guard!", "Turn!", "Parry!", "Dodge!", "Spin!", "Ha", "THRUST!" ]
def attack
ATTACKS.inject(0) { |dmg, word| dmg + yield(word) }
end
end
person = Character.default
puts person.battle_cry
num_attacks = 0;
damage = person.attack do |saying|
puts saying
num_attacks += 1
rand(3)
end
puts "#{damage} points of damage done in #{num_attacks} attacks"
In the above example, attack uses the yield keyword to call the block passed
to it. When we call attack, then, the local variable num_attacks is still
in scope in the block we pass it (delimited here by do ... end), so we can
increment it. attack is able to pass values into the block, here
they are captured into the saying variable. The block also passes values
back to the method, which show up as the return value of yield.
The word lambda in ruby usually means the lambda keyword, which is used
to make blocks into freestanding, function like objects (which themselves are usually
referred to as lambdas, procs, or Procs).
bounce = lambda { |thing| puts "I'm bouncing a #{thing}" }
bounce["ball"]
bounce["frog"]
So I think what you're asking is whether you can pass a Proc in place of a Hash
for an argument to a method. And the answer is "it depends". If the method only
ever uses the #[] method, then yes:
class Character
attr_accessor :stats
def set_stats(stats)
#stats = stats
end
end
frank = Character.new("Victor Frankenstein")
frank.set_stats({ :str => 7, :dex => 14, :con => 9, :int => 19, :wis => 7, :cha => 11 })
monster = Character.new("Frankenstein's Monster")
monster.set_stats(lambda do |stat_name|
rand(20)
end)
However, it might use some other Hash specific methods, or call the same key multiple times,
which can produce weird results:
monster = Character.new("Frankenstein's Monster")
monster.set_stats(lambda do |stat_name|
rand(20)
end)
monster.stats[:dex] #=> 19
monster.stats[:dex] #=> 1
In which case, you may be better off caching the requests in an intermediate hash. This is fairly easy,
since a Hash can have an initializer block. So if we change the above to:
monster.set_stats(Hash.new do |stats_hash, stat_name|
stats_hash[stat_name] = rand(20)
end)
monster.stats[:dex] #=> 3
monster.stats[:dex] #=> 3
The results are cached in the hash
To see more about Hash block initializers, see ri Hash::new:
-------------------------------------------------------------- Hash::new
Hash.new => hash
Hash.new(obj) => aHash
Hash.new {|hash, key| block } => aHash
------------------------------------------------------------------------
Returns a new, empty hash. If this hash is subsequently accessed
by a key that doesn't correspond to a hash entry, the value
returned depends on the style of new used to create the hash. In
the first form, the access returns nil. If obj is specified, this
single object will be used for all default values. If a block is
specified, it will be called with the hash object and the key, and
should return the default value. It is the block's responsibility
to store the value in the hash if required.
h = Hash.new("Go Fish")
h["a"] = 100
h["b"] = 200
h["a"] #=> 100
h["c"] #=> "Go Fish"
# The following alters the single default object
h["c"].upcase! #=> "GO FISH"
h["d"] #=> "GO FISH"
h.keys #=> ["a", "b"]
# While this creates a new default object each time
h = Hash.new { |hash, key| hash[key] = "Go Fish: #{key}" }
h["c"] #=> "Go Fish: c"
h["c"].upcase! #=> "GO FISH: C"
h["d"] #=> "Go Fish: d"
h.keys #=> ["c", "d"]