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.
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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.

 

Ruby Variables

Variables are the memory locations which hold any data to be used by any program.

There are five types of variables supported by Ruby. You already have gone through a small description of these variables in previous chapter as well. These five types of variables are explained in this chapter.

Ruby Global Variables:

Global variables begin with $. Uninitialized global variables have the value nil and produce warnings with the -w option.

Assignment to global variables alters global status. It is not recommended to use global variables. They make programs cryptic.

Here is an example showing usage of global variable.

#!/usr/bin/ruby

$global_variable = 10
class Class1
  def print_global
     puts "Global variable in Class1 is #$global_variable"
  end
end
class Class2
  def print_global
     puts "Global variable in Class2 is #$global_variable"
  end
end

class1obj = Class1.new
class1obj.print_global
class2obj = Class2.new
class2obj.print_global

Here $global_variable is a global variable. This will produce the following result:

NOTE: In Ruby you CAN access value of any variable or constant by putting a hash (#) character just before that variable or constant.

Global variable in Class1 is 10
Global variable in Class2 is 10

Ruby Instance Variables:

Instance variables begin with @. Uninitialized instance variables have the value nil and produce warnings with the -w option.

Here is an example showing usage of Instance Variables.

#!/usr/bin/ruby

class Customer
   def initialize(id, name, addr)
      @cust_id=id
      @cust_name=name
      @cust_addr=addr
   end
   def display_details()
      puts "Customer id #@cust_id"
      puts "Customer name #@cust_name"
      puts "Customer address #@cust_addr"
    end
end

# Create Objects
cust1=Customer.new("1", "John", "Wisdom Apartments, Ludhiya")
cust2=Customer.new("2", "Poul", "New Empire road, Khandala")

# Call Methods
cust1.display_details()
cust2.display_details()

Here, @cust_id, @cust_name and @cust_addr are instance variables. This will produce the following result:

Customer id 1
Customer name John
Customer address Wisdom Apartments, Ludhiya
Customer id 2
Customer name Poul
Customer address New Empire road, Khandala

Ruby Class Variables:

Class variables begin with @@ and must be initialized before they can be used in method definitions.

Referencing an uninitialized class variable produces an error. Class variables are shared among descendants of the class or module in which the class variables are defined.

Overriding class variables produce warnings with the -w option.

Here is an example showing usage of class variable:

#!/usr/bin/ruby

class Customer
   @@no_of_customers=0
   def initialize(id, name, addr)
      @cust_id=id
      @cust_name=name
      @cust_addr=addr
      @@no_of_customers += 1
   end
   def display_details()
      puts "Customer id #@cust_id"
      puts "Customer name #@cust_name"
      puts "Customer address #@cust_addr"
    end
    def total_no_of_customers()
       puts "Total number of customers: #@@no_of_customers"
    end
end

# Create Objects
cust1=Customer.new("1", "John", "Wisdom Apartments, Ludhiya")
cust2=Customer.new("2", "Poul", "New Empire road, Khandala")

# Call Methods
cust1.total_no_of_customers()
cust2.total_no_of_customers()

Here @@no_of_customers is a class variable. This will produce the following result:

Total number of customers: 1
Total number of customers: 2

Ruby Local Variables:

Local variables begin with a lowercase letter or _. The scope of a local variable ranges from class, module, def, or do to the corresponding end or from a block’s opening brace to its close brace {}.

When an uninitialized local variable is referenced, it is interpreted as a call to a method that has no arguments.

Assignment to uninitialized local variables also serves as variable declaration. The variables start to exist until the end of the current scope is reached. The lifetime of local variables is determined when Ruby parses the program.

In the above example local variables are id, name and addr.

Ruby Constants:

Constants begin with an uppercase letter. Constants defined within a class or module can be accessed from within that class or module, and those defined outside a class or module can be accessed globally.

Constants may not be defined within methods. Referencing an uninitialized constant produces an error. Making an assignment to a constant that is already initialized produces a warning.

#!/usr/bin/ruby

class Example
   VAR1 = 100
   VAR2 = 200
   def show
       puts "Value of first Constant is #{VAR1}"
       puts "Value of second Constant is #{VAR2}"
   end
end

# Create Objects
object=Example.new()
object.show

Here VAR1 and VAR2 are constant. This will produce the following result:

Value of first Constant is 100
Value of second Constant is 200

Ruby Pseudo-Variables:

They are special variables that have the appearance of local variables but behave like constants. You can not assign any value to these variables.

  • self: The receiver object of the current method.
  • true: Value representing true.
  • false: Value representing false.
  • nil: Value representing undefined.
  • __FILE__: The name of the current source file.
  • __LINE__: The current line number in the source file.

Courtesy: Tutorials Point

git reset

If git revert is a “safe” way to undo changes, you can think of git reset as thedangerous method. When you undo with git reset(and the commits are no longer referenced by any ref or the reflog), there is no way to retrieve the original copy—it is a permanent undo. Care must be taken when using this tool, as it’s one of the only Git commands that has the potential to lose your work.

Like git checkout, git reset is a versatile command with many configurations. It can be used to remove committed snapshots, although it’s more often used to undo changes in the staging area and the working directory. In either case, it should only be used to undolocal changes—you should never reset snapshots that have been shared with other developers.

Usage:

git reset <file>

Remove the specified file from the staging area, but leave the working directory unchanged. This unstages a file without overwriting any changes.

git reset

Reset the staging area to match the most recent commit, but leave the working directory unchanged. This unstages all files without overwriting any changes, giving you the opportunity to re-build the staged snapshot from scratch.

git reset --hard

Reset the staging area and the working directory to match the most recent commit. In addition to unstaging changes, the --hard flag tells Git to overwrite all changes in the working directory, too. Put another way: this obliterates all uncommitted changes, so make sure you really want to throw away your local developments before using it.

git reset <commit>

Move the current branch tip backward to <commit>, reset the staging area to match, but leave the working directory alone. All changes made since <commit> will reside in the working directory, which lets you re-commit the project history using cleaner, more atomic snapshots.

git reset --hard <commit>

Move the current branch tip backward to <commit> and reset both the staging area and the working directory to match. This obliterates not only the uncommitted changes, but all commits after <commit>, as well.

Discussion

All of the above invocations are used to remove changes from a repository. Without the --hard flag, git reset is a way to clean up a repository by unstaging changes or uncommitting a series of snapshots and re-building them from scratch. The --hard flag comes in handy when an experiment has gone horribly wrong and you need a clean slate to work with.

Whereas reverting is designed to safely undo a public commit, git reset is designed to undo local changes. Because of their distinct goals, the two commands are implemented differently: resetting completely removes a changeset, whereas revertingmaintains the original changeset and uses a new commit to apply the undo.

Git Tutorial: Revert vs Reset

Don’t Reset Public History

You should never use git reset <commit> when any snapshots after<commit> have been pushed to a public repository. After publishing a commit, you have to assume that other developers are reliant upon it.

Removing a commit that other team members have continued developing poses serious problems for collaboration. When they try to sync up with your repository, it will look like a chunk of the project history abruptly disappeared. The sequence below demonstrates what happens when you try to reset a public commit. The origin/master branch is the central repository’s version of your localmaster branch.

Git Tutorial: Resetting an Public Commit

As soon as you add new commits after the reset, Git will think that your local history has diverged from origin/master, and the merge commit required to synchronize your repositories is likely to confuse and frustrate your team.

The point is, make sure that you’re using git reset <commit> on a local experiment that went wrong—not on published changes. If you need to fix a public commit, the git revert command was designed specifically for this purpose.

Examples

Unstaging a File

The git reset command is frequently encountered while preparing the staged snapshot. The next example assumes you have two files called hello.py and main.py that you’ve already added to the repository.

# Edit both hello.py and main.py

# Stage everything in the current directory
git add .

# Realize that the changes in hello.py and main.py
# should be committed in different snapshots

# Unstage main.py
git reset main.py

# Commit only hello.py
git commit -m "Make some changes to hello.py"

# Commit main.py in a separate snapshot
git add main.py
git commit -m "Edit main.py"

As you can see, git reset helps you keep your commits highly-focused by letting you unstage changes that aren’t related to the next commit.

Removing Local Commits

The next example shows a more advanced use case. It demonstrates what happens when you’ve been working on a new experiment for a while, but decide to completely throw it away after committing a few snapshots.

# Create a new file called `foo.py` and add some code to it

# Commit it to the project history
git add foo.py
git commit -m "Start developing a crazy feature"

# Edit `foo.py` again and change some other tracked files, too

# Commit another snapshot
git commit -a -m "Continue my crazy feature"

# Decide to scrap the feature and remove the associated commits
git reset --hard HEAD~2

The git reset HEAD~2 command moves the current branch backward by two commits, effectively removing the two snapshots we just created from the project history. Remember that this kind of reset should only be used on unpublished commits. Never perform the above operation if you’ve already pushed your commits to a shared repository.

git rebase

Rebasing is the process of moving a branch to a new base commit. The general process can be visualized as the following:

Git Tutorial: Rebase to maintain a linear project history.

From a content perspective, rebasing really is just moving a branch from one commit to another. But internally, Git accomplishes this by creating new commits and applying them to the specified base—it’s literally rewriting your project history. It’s very important to understand that, even though the branch looks the same, it’s composed of entirely new commits.

Usage

git rebase <base>

Rebase the current branch onto <base>, which can be any kind of commit reference (an ID, a branch name, a tag, or a relative reference to HEAD).

Discussion

The primary reason for rebasing is to maintain a linear project history. For example, consider a situation where the master branch has progressed since you started working on a feature:

Git Rebase Branch onto Master

You have two options for integrating your feature into the masterbranch: merging directly or rebasing and then merging. The former option results in a 3-way merge and a merge commit, while the latter results in a fast-forward merge and a perfectly linear history. The following diagram demonstrates how rebasing onto master facilitates a fast-forward merge.

Git Tutorial: Fast-forward merge

Rebasing is a common way to integrate upstream changes into your local repository. Pulling in upstream changes with git merge results in a superfluous merge commit every time you want to see how the project has progressed. On the other hand, rebasing is like saying, “I want to base my changes on what everybody has already done.”

Don’t Rebase Public History

As we’ve discussed with git commit --amend and git reset, you should never rebase commits that have been pushed to a public repository. The rebase would replace the old commits with new ones, and it would look like that part of your project history abruptly vanished.

Courtesy: https://www.atlassian.com/git/tutorials/