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Is_a?

As explained in The Well-Grounded Rubyist, Fourth Edition

is_a? is a Ruby method that tests whether an object is an instance of a specified class or one of its subclasses.

is_a? checks whether an object belongs to a specified class or to one of that class’s ancestral classes. It follows Ruby’s inheritance hierarchy from specific classes toward general ones. section 3.8

>> mag = Magazine.new
=>  #<Magazine:0x000000012d972100>
>> mag.is_a?(Magazine)
=> true
>> mag.is_a?(Publication)
=> true

A Magazine instance reports true for both Magazine and its ancestor class Publication. section 3.8

Checking an inheritance relationship

If Magazine inherits from Publication, mag.is_a?(Magazine) and mag.is_a?(Publication) both return true. The test describes class ancestry, not every capability the object may acquire individually. section 3.8

How The Well-Grounded Rubyist, Fourth Edition covers it

How deep it goes
Defines class-ancestry testing and demonstrates it with a short example, without detailing method-dispatch internals.
Examples
Uses Magazine and Publication objects to test direct class and ancestral-class membership.
What it assumes
Assumes familiarity with Ruby objects and classes while grounding the test in inheritance relationships.
What you can do after
Evaluate whether an object belongs to a specified class hierarchy.
How it connects
Connects objects with classes, subclasses, superclasses, inheritance, and individually added methods.
Cover of The Well-Grounded Rubyist, Fourth Edition
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This explainer was generated by Manning's knowledge graph from the text of The Well-Grounded Rubyist, Fourth Edition. The section links point to the parts of the book it draws on.