Mocking and stubbing in Ruby - Step-by-Step Execution

Concept Flow - Mocking and stubbing

Start Test

↓

Create Stub

↓

Call Method

↓

Return Stubbed Value

↓

Create Mock

↓

Expect Method Call

↓

Verify Mock

↓

End Test

The test starts by creating stubs to replace methods with fixed returns, then mocks to expect calls, and finally verifies those expectations before ending.

Execution Sample

Ruby

class User
  def name; 'Alice'; end
end

user = User.new
allow(user).to receive(:name).and_return('Bob')
puts user.name

This code replaces the User's name method to always return 'Bob' instead of 'Alice'.

Execution Table

Step	Action	Method Called	Stub/Mock Setup	Return Value	Output
1	Create User instance	-	-	-	-
2	Stub 'name' method on user	name	Stub returns 'Bob'	-	-
3	Call 'name' method on user	name	-	Bob	Bob
4	End of test	-	-	-	-

💡 Test ends after method call returns stubbed value 'Bob'

Variable Tracker

Variable	Start	After Stub Setup	After Method Call	Final
user.name	Alice	Stubbed to 'Bob'	Returns 'Bob'	Bob

Key Moments - 2 Insights

Why does user.name return 'Bob' instead of 'Alice'?

Because in step 2 of the execution_table, the 'name' method is stubbed to always return 'Bob', overriding the original method.

What is the difference between a stub and a mock here?

A stub replaces a method to return a fixed value (step 2), while a mock would also check if the method was called as expected (not shown in this simple example).

Visual Quiz - 3 Questions

Look at the execution_table, what is the return value of user.name at step 3?

A 'Alice'

B 'Bob'

C nil

D Error

💡 Hint

Check the 'Return Value' column at step 3 in the execution_table.

At which step is the stub for the 'name' method created?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Stub/Mock Setup' column to find when the stub is applied.

If we remove the stub setup in step 2, what would user.name return at step 3?

A 'Alice'

B nil

C 'Bob'

D Error

💡 Hint

Refer to variable_tracker for user.name initial value before stubbing.

Concept Snapshot

Mocking and stubbing in Ruby:
- Stub replaces a method to return fixed value.
- Mock sets expectations on method calls.
- Use 'allow(obj).to receive(:method).and_return(value)' for stubbing.
- Use 'expect(obj).to receive(:method)' for mocking.
- Verify mocks to ensure methods were called.
- Helps isolate tests from real dependencies.

Full Transcript

This visual trace shows how mocking and stubbing work in Ruby tests. First, a User object is created with a name method returning 'Alice'. Then, the name method is stubbed to return 'Bob' instead. When calling user.name, the stubbed value 'Bob' is returned instead of the original 'Alice'. This helps tests control method outputs. Mocks would also check if methods are called as expected, but this example focuses on stubbing. The variable tracker shows how user.name changes from 'Alice' to 'Bob' after stubbing. The key moments clarify why the stub changes the return value and the difference between mocks and stubs. The quiz questions help reinforce understanding by referencing the execution steps and variable states.