ruby on rails - Seeing if a database line exists matching two foreign keys -

I have a model called Alpha, another model, Beta has foreign keys of alpha and gamma. I want to see if beta is present for a given alpha and gamma match, so I tried to do it for the alphabet named 'A' and a gamma named 'G' but it does not work because it For some reason to know the gamma for every beta, I know that I can put it in a dirty loop, but it seems very non-ROR.

  a.betas.gamma.includes? (G)    I also tried various forms of  a.betas.find (: gamma => g)   
 They also do not work, do I just accept to take everyone into the Beta model and make Beta class system? This is remarkably inconvenient.  
   
  If I think right:  
  class alpha < & Lt; ActiveRecord :: Base is - Medium: Betas End Class Beta & lt; & Lt; ActiveRecord :: Base is_to: alpha is_to: Gamma and Class Gamma & lt; & Lt; ActiveRecord :: Base is has_many: betas end    Can you add : mam: gammas, through = & gt; : Beta for Alpha class? If so, try:  
  a.gammas.exists? (G)    

 


  







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