Method: ActiveRecord::Base.import

Defined in:
lib/activerecord-import/import.rb

.import(*args) ⇒ Object

Imports a collection of values to the database.

This is more efficient than using ActiveRecord::Base#create or ActiveRecord::Base#save multiple times. This method works well if you want to create more than one record at a time and do not care about having ActiveRecord objects returned for each record inserted.

This can be used with or without validations. It does not utilize the ActiveRecord::Callbacks during creation/modification while performing the import.

Usage

Model.import array_of_models
Model.import column_names, array_of_values
Model.import column_names, array_of_values, options

Model.import array_of_models

With this form you can call import passing in an array of model objects that you want updated.

Model.import column_names, array_of_values

The first parameter column_names is an array of symbols or strings which specify the columns that you want to update.

The second parameter, array_of_values, is an array of arrays. Each subarray is a single set of values for a new record. The order of values in each subarray should match up to the order of the column_names.

Model.import column_names, array_of_values, options

The first two parameters are the same as the above form. The third parameter, options, is a hash. This is optional. Please see below for what options are available.

Options

  • validate – true|false, tells import whether or not to use \
    ActiveRecord validations. Validations are enforced by default.
  • on_duplicate_key_update – an Array or Hash, tells import to \
    use MySQL's ON DUPLICATE KEY UPDATE ability. See On Duplicate\
    Key Update below.
  • synchronize – an array of ActiveRecord instances for the model that you are currently importing data into. This synchronizes existing model instances in memory with updates from the import.
  • timestamps – true|false, tells import to not add timestamps \ (if false) even if record timestamps is disabled in ActiveRecord::Base
  • +recursive – true|false, tells import to import all autosave association if the adapter supports setting the primary keys of the newly imported objects.

Arraying your arguments – Ruby

The list of parameters passed to an object is, in fact, available as a list. To do this, we use what is called the splat operator – which is just an asterisk (*).

The splat operator is used to handle methods which have a variable parameter list. Let’s use it to create an add method that can handle any number of parameters.

We use the inject method to iterate over arguments, which is covered in the chapter on Collections. It isn’t directly relevant to this lesson, but do look it up if it piques your interest.

Example Code:

def add(*numbers)
  numbers.inject(0) { |sum, number| sum + number }
end

puts add(1)
puts add(1, 2)
puts add(1, 2, 3)
puts add(1, 2, 3, 4)

The splat operator works both ways – you can use it to convert arrays to parameter lists as easily as we just converted a parameter list to an array.

Inject in Ruby

The syntax for the inject method is as follows:

inject (value_initial) { |result_memo, object| block }

Let’s solve the above example i.e.

[1, 2, 3, 4].inject(0) { |result, element| result + element }

which gives the 10 as the output.

So, before starting let’s see what are the values stored in each variables:

result = 0 The zero came from inject(value) which is 0

element = 1 It is first element of the array.

Okey!!! So, let’s start understanding the above example

Step :1 [1, 2, 3, 4].inject(0) { |0, 1| 0 + 1 }

Step :2 [1, 2, 3, 4].inject(0) { |1, 2| 1 + 2 }

Step :3 [1, 2, 3, 4].inject(0) { |3, 3| 3 + 3 }

Step :4 [1, 2, 3, 4].inject(0) { |6, 4| 6 + 4 }

Step :5 [1, 2, 3, 4].inject(0) { |10, Now no elements left in the array, so it'll return 10 from this step| }

Here Bold-Italic values are elements fetch from array and the simply Bold values are the resultant values.

I hope that you understand the working of the #inject method of the #ruby.

 

Active Record Basics

1 What is Active Record?

Active Record is the M in MVC – the model – which is the layer of the system responsible for representing business data and logic. Active Record facilitates the creation and use of business objects whose data requires persistent storage to a database. It is an implementation of the Active Record pattern which itself is a description of an Object Relational Mapping system.

1.1 The Active Record Pattern

Active Record was described by Martin Fowler in his book Patterns of Enterprise Application Architecture. In Active Record, objects carry both persistent data and behavior which operates on that data. Active Record takes the opinion that ensuring data access logic as part of the object will educate users of that object on how to write to and read from the database.

1.2 Object Relational Mapping

Object Relational Mapping, commonly referred to as its abbreviation ORM, is a technique that connects the rich objects of an application to tables in a relational database management system. Using ORM, the properties and relationships of the objects in an application can be easily stored and retrieved from a database without writing SQL statements directly and with less overall database access code.

1.3 Active Record as an ORM Framework

Active Record gives us several mechanisms, the most important being the ability to:

  • Represent models and their data.
  • Represent associations between these models.
  • Represent inheritance hierarchies through related models.
  • Validate models before they get persisted to the database.
  • Perform database operations in an object-oriented fashion.

2 Convention over Configuration in Active Record

When writing applications using other programming languages or frameworks, it may be necessary to write a lot of configuration code. This is particularly true for ORM frameworks in general. However, if you follow the conventions adopted by Rails, you’ll need to write very little configuration (in some cases no configuration at all) when creating Active Record models. The idea is that if you configure your applications in the very same way most of the time then this should be the default way. Thus, explicit configuration would be needed only in those cases where you can’t follow the standard convention.

2.1 Naming Conventions

By default, Active Record uses some naming conventions to find out how the mapping between models and database tables should be created. Rails will pluralize your class names to find the respective database table. So, for a class Book, you should have a database table called books. The Rails pluralization mechanisms are very powerful, being capable of pluralizing (and singularizing) both regular and irregular words. When using class names composed of two or more words, the model class name should follow the Ruby conventions, using the CamelCase form, while the table name must contain the words separated by underscores. Examples:

  • Database Table – Plural with underscores separating words (e.g., book_clubs).
  • Model Class – Singular with the first letter of each word capitalized (e.g., BookClub).
Model / Class Table / Schema
Article articles
LineItem line_items
Deer deers
Mouse mice
Person people

2.2 Schema Conventions

Active Record uses naming conventions for the columns in database tables, depending on the purpose of these columns.

  • Foreign keys – These fields should be named following the pattern singularized_table_name_id (e.g., item_id, order_id). These are the fields that Active Record will look for when you create associations between your models.
  • Primary keys – By default, Active Record will use an integer column named id as the table’s primary key. When using Active Record Migrations to create your tables, this column will be automatically created.

There are also some optional column names that will add additional features to Active Record instances:

  • created_at – Automatically gets set to the current date and time when the record is first created.
  • updated_at – Automatically gets set to the current date and time whenever the record is updated.
  • lock_version – Adds optimistic locking to a model.
  • type – Specifies that the model uses Single Table Inheritance.
  • (association_name)_type – Stores the type for polymorphic associations.
  • (table_name)_count – Used to cache the number of belonging objects on associations. For example, a comments_count column in an Article class that has many instances of Comment will cache the number of existent comments for each article.

While these column names are optional, they are in fact reserved by Active Record. Steer clear of reserved keywords unless you want the extra functionality. For example, type is a reserved keyword used to designate a table using Single Table Inheritance (STI). If you are not using STI, try an analogous keyword like “context”, that may still accurately describe the data you are modeling.

3 Creating Active Record Models

It is very easy to create Active Record models. All you have to do is to subclass the ApplicationRecord class and you’re good to go:

class Product < ApplicationRecord
end

This will create a Product model, mapped to a products table at the database. By doing this you’ll also have the ability to map the columns of each row in that table with the attributes of the instances of your model. Suppose that the products table was created using an SQL statement like:

CREATE TABLE products (
   id int(11) NOT NULL auto_increment,
   name varchar(255),
   PRIMARY KEY  (id)
);

Following the table schema above, you would be able to write code like the following:

p = Product.new
p.name = "Some Book"
puts p.name # "Some Book"

4 Overriding the Naming Conventions

What if you need to follow a different naming convention or need to use your Rails application with a legacy database? No problem, you can easily override the default conventions.

ApplicationRecord inherits from ActiveRecord::Base, which defines a number of helpful methods. You can use the ActiveRecord::Base.table_name= method to specify the table name that should be used:

class Product < ApplicationRecord
  self.table_name = "my_products"
end

If you do so, you will have to define manually the class name that is hosting the fixtures (my_products.yml) using the set_fixture_class method in your test definition:

class ProductTest < ActiveSupport::TestCase
  set_fixture_class my_products: Product
  fixtures :my_products
  ...
end

It’s also possible to override the column that should be used as the table’s primary key using the ActiveRecord::Base.primary_key= method:

class Product < ApplicationRecord
  self.primary_key = "product_id"
end

5 CRUD: Reading and Writing Data

CRUD is an acronym for the four verbs we use to operate on data: Create, Read, Update and Delete. Active Record automatically creates methods to allow an application to read and manipulate data stored within its tables.

5.1 Create

Active Record objects can be created from a hash, a block or have their attributes manually set after creation. The new method will return a new object while create will return the object and save it to the database.

For example, given a model User with attributes of name and occupation, the create method call will create and save a new record into the database:

user = User.create(name: "David", occupation: "Code Artist")

Using the new method, an object can be instantiated without being saved:

user = User.new
user.name = "David"
user.occupation = "Code Artist"

A call to user.save will commit the record to the database.

Finally, if a block is provided, both create and new will yield the new object to that block for initialization:

user = User.new do |u|
  u.name = "David"
  u.occupation = "Code Artist"
end

5.2 Read

Active Record provides a rich API for accessing data within a database. Below are a few examples of different data access methods provided by Active Record.

# return a collection with all users
users = User.all
# return the first user
user = User.first
# return the first user named David
david = User.find_by(name: 'David')
# find all users named David who are Code Artists and sort by created_at in reverse chronological order
users = User.where(name: 'David', occupation: 'Code Artist').order(created_at: :desc)

You can learn more about querying an Active Record model in the Active Record Query Interfaceguide.

5.3 Update

Once an Active Record object has been retrieved, its attributes can be modified and it can be saved to the database.

user = User.find_by(name: 'David')
user.name = 'Dave'
user.save

A shorthand for this is to use a hash mapping attribute names to the desired value, like so:

user = User.find_by(name: 'David')
user.update(name: 'Dave')

This is most useful when updating several attributes at once. If, on the other hand, you’d like to update several records in bulk, you may find the update_all class method useful:

User.update_all "max_login_attempts = 3, must_change_password = 'true'"

5.4 Delete

Likewise, once retrieved an Active Record object can be destroyed which removes it from the database.

user = User.find_by(name: 'David')
user.destroy

6 Validations

Active Record allows you to validate the state of a model before it gets written into the database. There are several methods that you can use to check your models and validate that an attribute value is not empty, is unique and not already in the database, follows a specific format and many more.

Validation is a very important issue to consider when persisting to the database, so the methods save and update take it into account when running: they return false when validation fails and they didn’t actually perform any operation on the database. All of these have a bang counterpart (that is, save! and update!), which are stricter in that they raise the exception ActiveRecord::RecordInvalid if validation fails. A quick example to illustrate:

class User < ApplicationRecord
  validates :name, presence: true
end
user = User.new
user.save  # => false
user.save! # => ActiveRecord::RecordInvalid: Validation failed: Name can't be blank

You can learn more about validations in the Active Record Validations guide.

7 Callbacks

Active Record callbacks allow you to attach code to certain events in the life-cycle of your models. This enables you to add behavior to your models by transparently executing code when those events occur, like when you create a new record, update it, destroy it and so on. You can learn more about callbacks in the Active Record Callbacks guide.

8 Migrations

Rails provides a domain-specific language for managing a database schema called migrations. Migrations are stored in files which are executed against any database that Active Record supports using rake. Here’s a migration that creates a table:

class CreatePublications < ActiveRecord::Migration[5.0]
  def change
    create_table :publications do |t|
      t.string :title
      t.text :description
      t.references :publication_type
      t.integer :publisher_id
      t.string :publisher_type
      t.boolean :single_issue
      t.timestamps
    end
    add_index :publications, :publication_type_id
  end
end

Rails keeps track of which files have been committed to the database and provides rollback features. To actually create the table, you’d run rails db:migrate and to roll it back, rails db:rollback.

Note that the above code is database-agnostic: it will run in MySQL, PostgreSQL, Oracle and others. You can learn more about migrations in the Active Record Migrations guide.

Courtesy: guides.rubyonrails.org

Login loop issue on Ubuntu

Had an issue with Ubuntu 14.04 version where in login into the system would result in the screen going through various screens and end up back at login page. I had previously had the same issue but was able to resolve it with the help of my friend. This time I thought I’d try to fix this myself and was able to faster than I thought.

Here’s how I resolved it after going through a few solutions :-

So basically lightdm is the display manager which comes by default with 14.04. So when you google for lightdm here’s what you find …

LightDM is an X display manager that aims to be lightweight, fast, extensible and multi-desktop. It uses various front-ends to draw login interfaces, also called Greeters.

Basically this package manages the login interface.To me that’s not a show stopper, in fact all my work starts after login.So I just thought I’d try another display manager. There are different display managers that work with ubuntu, another one being gdm. I just ran the following command to remove lightdm and install gdm.

CNTRL + ALT + F1 launches the terminal window even when user is’nt logged in.

sudo apt-get purge lightdm && sudo apt-get install gdm

This fixed my issue. Now I’m able to login to my machine without a prob. Case closed!

Comparison Query Operators

For details on specific operator, including syntax and examples, click on the specific operator to go to its reference page.

For comparison of different BSON type values, see the specified BSON comparison order.

Name Description
$eq Matches values that are equal to a specified value.
$gt Matches values that are greater than a specified value.
$gte Matches values that are greater than or equal to a specified value.
$lt Matches values that are less than a specified value.
$lte Matches values that are less than or equal to a specified value.
$ne Matches all values that are not equal to a specified value.
$in Matches any of the values specified in an array.
$nin Matches none of the values specified in an array.

How to remove an application completely from linux

  • apt-get remove packagename will remove the binaries, but not the configuration or data files of the package packagename. It will also leave dependencies installed with it on installation time untouched.
  • apt-get purge packagename  or apt-get remove --purge packagenamewill remove about everything regarding the package packagename, but not the dependencies installed with it on installation. Both commands are equivalent.Particularly useful when you want to ‘start all over’ with an application because you messed up the configuration. However, it does not remove configuration or data files residing in users home directories, usually in hidden folders there. There is no easy way to get those removed as well.
  • apt-get autoremove  removes orphaned packages, i.e. installed packages that used to be installed as an dependency, but aren’t any longer. Use this after removing a package which had installed dependencies you’re no longer interested in.
  • aptitude remove packagename  or aptitude purge packagename (likewise)will also attempt to remove other packages which were required by packagename on but are not required by any remaining packages.

And many more exist. Lower-level dpkg-commands can be used (advanced), or GUI tools like Muon, Synaptic, Software Center, etc. There’s no single ‘correct way’ of removing applications or performing other tasks interacting with your package management.

The list you found are just examples. Make sure you understand the meanings and try out what it wants to do before accepting the action (you need to press Y before it actually performs the actions as proposed).

The asterisk version in the question is probably wrong; apt-get accepts a regular expression and not a glob pattern as the shell. So what happens with

sudo apt-get remove application*

is the following:

  1. The shell tries to expand application* looking at the files in the current directory. If (as is normally the case) it finds nothing, it returns the glob pattern unaltered (supposing bashwith default behavior here — zsh will error out).
  2. apt-get will remove the packages whose name contains a string that satisfies the regular expression application*, that is, applicatio followed by an arbitrary number of n: applicatio, application, applicationn, libapplicatio, etc.
  3. To see how this can be dangerous, try (without root for double safety) apt-get -s remove "wine*" (-s will simulate the thing instead of doing it) — it will say is going to remove all packages that has “win” in their name and the dependant, almost the entire system…

Probably, the command that was meant is really

 sudo apt-get remove "^application.*"

(note the quotes and the dot) which will remove all packages whose name starts with application.

These commands,

sudo updatedb                  # 

are completely outside the scope of the package management. Do not remove files belonging to packages without using the package manager! It will get confused and is the wrong way to do things.

If you don’t know to which package a file belongs, try this:

dpkg -S /path/to/file