I have a table named students.
I need to get email field from specific students
for example, I get the following output in IRB
Student.all
Student Load (0.1ms) SELECT "students".* FROM "students"
=> [#<Student id: 1, name: "Bob", grade: 1, email_address: "bob#school.com", created_at: "2014-03-27 08:55:51", updated_at: "2014-03-27 08:55:51">, #<Student id: 2, name: "Neo", grade: 1, email_address: "robert#neo.com", created_at: "2014-03-27 08:56:05", updated_at: "2014-03-27 08:56:05">, #<Student id: 3, name: "Phil", grade: 3, email_address: "phil#school.com", created_at: "2014-03-27 08:56:21", updated_at: "2014-03-27 08:56:21">]
now I need to get email addresses of grade 1 students.
How could I get it?
I solved it,
it can be done by using pluck function:
mail_addresses = Student.where(grade: 1).pluck(:email_address)
mail_addresses.each do|a|
puts a
end
Output :
bob#school.com
robert#neo.com
=> ["bob#school.com", "robert#neo.com"]
Voila!
I think that it will help you.
There are many way to write queries to get email addresses of student with grade 1.
Student.where(:grade => 1).map(&:email)
or
Student.where(:grade => 1).collect(&:email)
or
Student.where("grade = ?", 1).map(&:email)
or
Student.find(:all, :conditions => ["grade = ?", 1]).map(&:email)
you can use select also.
Student.where("grade = ?", 1).select("email").map(&:email)
always use rails query with where instead of find. rails query with `where' is working fast instead of find.
Related
I start to using pry in a rails console.
When I get a instance of a Rails model, the values are shown without line breaks like this:
pry(#<Class:0x1022f60e0>):1> first
=> #<Article id: 1, name: "What is Music", content: "Music is an art form in which the medium is sound o...", created_at: "2011-08-24 20:35:29", updated_at: "2011-08-24 20:37:22", published_at: "2011-05-13 23:00:00">
from http://railscasts.com/episodes/280-pry-with-rails?view=asciicast
Is there way to show the values with line breaks like this?
Article
id: 1
name: "What is Music"
content: "Music is an art form in which the medium is sound o..."
created_at: "2011-08-24 20:35:29"
updated_at: "2011-08-24 20:37:22"
published_at: "2011-05-13 23:00:00"
You could call .to_yaml on the model instance! It returns a string that's formatted almost exactly like you're requesting it to be.
Here are some examples of to_yaml output:
http://yaml4r.sourceforge.net/doc/page/examples.htm
I would recommend that you install awesome_print.
Add it to your Gemfile:
group :development do
gem 'awesome_print'
end
And install it with bundle install.
Now use ap to print it in the console:
pry(#<Class:0x1022f60e0>):1> ap first
#<Article:0x1022f60e0> {
:id => 1,
:name => "What is Music"
:content => "Music is an art form in which the medium is sound o..."
:created_at => "2011-08-24 20:35:29"
:updated_at => "2011-08-24 20:37:22"
:published_at => "2011-05-13 23:00:00"
}
I think, the below trick will work for you.
arup#linux-wzza:~/Rails/model_prac> rails c
Loading development environment (Rails 4.1.4)
2.1.2 :001 > Comment.first
Comment Load (0.4ms) SELECT "comments".* FROM "comments" ORDER BY "comments"."id" ASC LIMIT 1
=> #<Comment id: 1, value_old: "I am a good Boy.", value_new: "I am a bad Boy.", created_at: "2014-08-02 17:36:14", updated_at: "2014-08-02 18:21:42">
2.1.2 :002 > y Comment.first
Comment Load (0.4ms) SELECT "comments".* FROM "comments" ORDER BY "comments"."id" ASC LIMIT 1
--- !ruby/object:Comment
attributes:
id: 1
value_old: I am a good Boy.
value_new: I am a bad Boy.
created_at: 2014-08-02 17:36:14.249466000 Z
updated_at: 2014-08-02 18:21:42.511522000 Z
=> nil
2.1.2 :003 >
Rolify has a join table called user_roles that keeps track of all the relationships:
user_id role_id
But here's what's weird. When I empty this table of all data each user still has its role remembered.
I'm trying to manipulate the database in my specs, and it's impossible because of this spookiness.
Look at this from the console:
user = User.save
irb(main):022:0> user.roles # nothing weird, I have a before filter to make every new user a guest
=> #<ActiveRecord::Associations::CollectionProxy [#<Role id: 1, name: "guest", resource_id: nil, resource_type: nil, created_at: "2014-08-17 10:04:57", updated_at: "2014-08-17 10:04:57">]>
irb(main):023:0> UsersRole.all.each do |role| puts role.inspect end
=> [#<UsersRole user_id: 2, role_id: 1, id: 4>, #<UsersRole user_id: 2, role_id: 1, id: 5>]
irb(main):024:0> UsersRole.all.each do |role| role.delete end
=> [] # completely empty!
irb(main):026:0> user.roles # no way should this get anything
=> #<ActiveRecord::Associations::CollectionProxy [#<Role id: 1, name: "guest", resource_id: nil, resource_type: nil, created_at: "2014-08-17 10:04:57", updated_at: "2014-08-17 10:04:57">]>
I'd really appreciate some help into why this is happening.
you should reload your object after deleting the roles. rails caches association.
user = User.save
irb(main):022:0> user.roles
=> #<ActiveRecord::Associations::CollectionProxy [#<Role id: 1, name: "guest", resource_id: nil, resource_type: nil, created_at: "2014-08-17 10:04:57", updated_at: "2014-08-17 10:04:57">]>
irb(main):023:0> UsersRole.all.each do |role| puts role.inspect end
=> [#<UsersRole user_id: 2, role_id: 1, id: 4>, #<UsersRole user_id: 2, role_id: 1, id: 5>]
irb(main):024:0> UsersRole.all.each do |role| role.delete end
=> [] # completely empty!
irb(main):026:0> user.reload.roles
In the rails console, I do this:
input = Input.create :name => "foo"
=> #<Input id: 8, name: "foo", created_at: "2013-05-07 11:45:17", updated_at: "2013-05-07 11:45:17">
Input.all
=> [#<Input id: 8, name: "foo", created_at: "2013-05-07 11:45:17", updated_at: "2013-05-07 11:45:17">]
input
=> #<Input id: 8, name: "foo", created_at: "2013-05-07 11:45:17", updated_at: "2013-05-07 11:45:17">
input.destroy
=> #<Input id: 8, name: "foo", created_at: "2013-05-07 11:45:17", updated_at: "2013-05-07 11:45:17">
> Input.all
=> []
> input
=> #<Input id: 8, name: "foo", created_at: "2013-05-07 11:45:17", updated_at: "2013-05-07 11:45:17">
> input.reload
ActiveRecord::RecordNotFound: Couldn't find Input with id=8
> input
=> #<Input id: 8, name: "foo", created_at: "2013-05-07 11:45:17", updated_at: "2013-05-07 11:45:17">
What I'd really expect to see is something like:
> input
=> nil
The object is deleted from the database but the variable still exists and is still trying to point to it. What's going on?
The input variable stores a reference to the instance in memory. Destroying the record will remove the row from the database. Calling input.reload (docs) raises an exception when attempting to find the record but doesn't set the value of your variable to nil on your behalf.
This behavior can be useful in the span of a DELETE request in which you want to display information about the object you removed. For example:
class WidgetsController < ApplicationController
def destroy
#widget = Widget.find(params[:id])
#widget.destroy
respond_with #widget, notice: "You successfully removed #{#widget.name}"
end
end
The destroy method makes the SQL call to the database and destroys the row in the table that contains it. It does still allow you to manipulate the object in the application as long as it’s still in scope (i.e) the callbacks and
filters are allowed even after destroying the object.
It is better to use "delete" if we don't want the callbacks to be triggered or if we want better performance
you can use input.delete
In my project, the user will have many items, which have an onshelf_at attribute default to DateTime.now at its creation.
# item.rb
class Item < ActiveRecord::Base
before_create :calculate_onshelf_time
default_scope :order => 'items.onshelf_at DESC'
def calculate_onshelf_time
self.onshelf_at = DateTime.now
end
end
In the user model test, I tried to convince myself the retrieved order is indeed items.onshelf_at DESC. So I made the following snippet, but the result turned out to be reverse. (namely [#item1, #item2])
# spec/models/user_spec.rb
before :each do
#user = User.create(#attr)
#item1 = Factory(:item, :owner=>#user, :onshelf_at => 2.days.ago, :created_at => 2.days.ago)
#item2 = Factory(:item, :owner=>#user, :onshelf_at => 1.day.ago, :created_at => 1.day.ago)
end
it "should have the right items in the right order" do
#user.items.should == [#item2, #item1]
end
I checked the console, and found that onshelf_at wasn't listening to the instance initialization of Factory Girl. In its stead, it followed before_create rule, and valued to the time when test was run!
Failure/Error: #user.items.should == [#item2, #item1]
expected: [#<Item id: 2, description: "this is an item", img_link: "http://www.example.com/photos/some_pic.jpg", category_id: 5, onshelf: true, created_at: "2011-11-20 11:19:15", updated_at: "2011-11-21 11:19:15", onshelf_at: "2011-11-21 11:19:15", owner_id: 1>, #<Item id: 1, description: "this is an item", img_link: "http://www.example.com/photos/some_pic.jpg", category_id: 5, onshelf: true, created_at: "2011-11-19 11:19:15", updated_at: "2011-11-21 11:19:15", onshelf_at: "2011-11-21 11:19:15", owner_id: 1>]
got: [#<Item id: 1, description: "this is an item", img_link: "http://www.example.com/photos/some_pic.jpg", category_id: 5, onshelf: true, created_at: "2011-11-19 11:19:15", updated_at: "2011-11-21 11:19:15", onshelf_at: "2011-11-21 11:19:15", owner_id: 1>, #<Item id: 2, description: "this is an item", img_link: "http://www.example.com/photos/some_pic.jpg", category_id: 5, onshelf: true, created_at: "2011-11-20 11:19:15", updated_at: "2011-11-21 11:19:15", onshelf_at: "2011-11-21 11:19:15", owner_id: 1>] (using ==)
How can I fix this?
A quick fix would be to add a condition to your calculate_onshelf_time method:
self.onshelf_at = DateTime.now if self.onshelf_at.nil?
But - you don't even need all this. If you can (that is, if you have control over the schema), replace the onshelf_at attribute with a created_at column, which will automatically be set by Rails at the time of creation. If you're using migrations:
create_table :foo do |t|
# ...
t.timestamps
end
will add created_at and updated_at timestamps to the model.
I'm testing Rails 3 beta 4 on Ruby 1.9.2-head, and when I start a
console and do:
Game.first.to_sql
I get this error:
ArgumentError: wrong number of arguments (0 for 1)
I know it can find the Game record, because when I type:
Game.first
it returns:
=> #<Game id: 1, name: "Galaga", created_at: "2010-06-19 11:02:37",
updated_at: "2010-06-19 11:02:37">
What am I missing? I just want to make the to_sql work in a very simple
case.
.
When you run Game.first you are returning a Game object, not a ActiveRecord::Relation object as you are expecting.
To do what you're trying to do, you'll need to do:
Game.limit(1).to_sql
This lets you run it without to_sql and return the object as you expected it, although it will be in an array, which then you can run .first on it like you wanted anyways.
irb(main):004:0> Game.limit(1).to_sql
=> "SELECT `games`.* FROM `games` LIMIT 1"
irb(main):005:0> Game.limit(1).class
=> ActiveRecord::Relation
irb(main):006:0> Game.limit(1)
=> [#<Game id: 1, name: "Galaga", created_at: "2010-06-19 11:02:37", updated_at: "2010-06-19 11:02:37">]
irb(main):007:0> Game.limit(1).first
=> #<Game id: 1, name: "Galaga", created_at: "2010-06-19 11:02:37", updated_at: "2010-06-19 11:02:37">
When you dig into the source, when you run .first on an ActiveRecord::Relation it runs the following (which is the same as I showed you):
def find_first
if loaded?
#records.first
else
#first ||= limit(1).to_a[0]
end
end