[Rails] Defining Relationship ( Many to Many) for 3 Models in Rails

I have three models in Rails as : Curriculum, Grade and Topics.

The relationship scenario is like :

Curriculum 'International Baccalaureate(IB)' in grade '6' has_many Topics ( t1,t2,t3 and CompareFraction)

Curriculum 'CBSE' in grade '5' has_many Topics (t1,t2,t4,t5 and CompareFraction) [ A Topic 'Compare Fraction' will be taught in many different curriculums but maybe in different grades ]

A Grade, say 5 will itself be a part of all Curriculums like IB,CBSE.

I need to store information such that for a Topic Compare Fraction, I can say:

It is taught in IB in grade 5

It is taught in CBSE in grade 4.

How can I set this up in Rails?

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