some of the common exceptions in ruby on rails and how to resolve them.

Everything is ruby is a method. Ruby methods are as of Human Language such as, merge, select, sort, map etc., which makes ruby one of the most used language today. It’s also difficult to debug in ruby.

What is Standard Error…?

The big daddy of all Ruby exceptions, the StandardError. StandardError is a superclass with many exception subclasses of its own, but like all errors, it descends from the Exception superclass. StandardErrors occur anytime a rescue clause catches an exception without an explicit Exception class specified.

Types of Errors and how to resolve them.

1. NoMethodError

undefined method `grade' for nil:NilClass
undefined method `<<' for {}:Hash

When a method called for a particular object is not present, the above error is thrown. The most common is nil:Nilclass

[1] pry(main)> nil.grade
NoMethodError: undefined method `grade' for nil:NilClass
[2] pry(main)>

Use try method for solving nil class errors. try returns nil in any case a method is called for nil object.

[2] pry(main)> nil.try(:something)
=> nil
[3] pry(main)>

2. NoMethodError (for private method)

private method `resume' called for #Student:0x007ff2f69a0218

The above happens when we try to access private method of a class from another class.
If we need to access a method of inherited class then use protected and access a method of different class make it public method.

3. NameError

undefined local variable or method `grade' for #
uninitialized constant Report 

The above error occurs when a variable is not initialised. This is similar to unknown variable error in C or C++.

You know, it's simple to resolve, just initialise it. grade = 0 or grade = "".

4. ActiveRecord::RecordNotFound

Couldn\'t find id=45

When the row or record having a particular 'id' is not present in database. This usually occurs when we use Student.find(params[:id]) .

We can use find_by and where which never throws exception, but returns nil, when record is not found in the database.

# does not throw exception
[2] pry(main)> Student.find_by_id(params[:id]) 
=> nil
[3] pry(main)> Student.find_by(id: params[:id])
=> nil
[4] pry(main)> Student.where(id: params[:id]).first
=> nil

5. TypeError

String can't be coerced into Fixnum
no implicit conversion of String into Integer

Say, we add a string and a number like "1" + 2 , TypeError occurs. There are String, FixNum, Array, Hash, Float datatypes.
Convert the object into same datatypes to perform any action.

[3] pry(main)> "1" + 2
TypeError: no implicit conversion of Fixnum into String
from (pry):3:in `+'
[4] pry(main)> "1".to_i + 2
=> 3
[5] pry(main)>

6. ArgumentError

wrong number of arguments (0 for 1)

It's simple, Arguments of calling and called method dont't match.

my_grade = student(college, grade)

def student(college) # Argument error
end

def student(college, grade = 0) # works fine as grade is assigned 0.
end

7. ActionView::Template::Error

The above occurs when there is no template for a particular action in a controller and there is no redirects to any other path or action.
Redirecting to required path or adding the required template for that particular action would solve the issue.

8. Routing Error

No route matches [GET] "/students"

You are accessing a wrong url or there has been no route initialised in config/routes .

Use rake routes to find out all the routes in the app.

9. Syntax Error

Most famous error of all in every language 🙂 . Look into "" (quotes) or a , (comma). It would be missing.

10. LoadError

 Could not load 'active_record/connection_adapters/_adapter'. 

This is not allow server to begin as there is no adapter in config/database.yml for connecting to database, it's a configuration issue.

Make sure that the adapter in config/database.yml is valid. mysql2, postgresql or sqlite3 and adding necessary adapter in the Gemfile.

11. URI::InvalidURIError (bad URI(is not URI?): ):

The above occurs when a URL is invalid or http:// or https:// is missing in the URL. This is most likely to happen in ENV files.
Check the URL, add http:// or https:// as required.

12. Net::ReadTimeout

When we make a API call a third party server from our app and the third party server does not respond, Net::ReadTimeout occurs.
Having a timeout for the API call is the solution. Use HTTParty for making a API call, I recommend this gem to use 🙂

13. ActiveRecord::PendingMigrationError

Especially for rails newbies, when there is a migration is added and it doesn't reflect in the schema, then run rake db:migrate .
This would solve the above. Note that this occurs only in DEV environment, likely to be suppressed in Production.

14. RunTimeError

Server cannot run because of some blockers such as Configuration issues or Syntax errors.
Depends on the scenario to debug.

Exception handling in ruby

Use rescue to handle exceptions and give a definite response.

def some_method
  puts "no exceptions till here"
  # something went wrong

  rescue => e
  puts "something went wrong"

  { status => "error",
    "message" => "#{e}"
  }
end