dynamically get rails association

What about just doing this?

customer = o.send(relationship)

You can use try() which will allow you manage any undefined method errors if the relationship doesn't exist.

relationship = 'customer'
foo = 'foo'

customer = o.try(relationship)
# > [
#     [0] #<Customer...

customer = o.try(foo)
# > nil

vs send():

customer = o.send(relationship)
# > [
#     [0] #<Customer... 

customer = o.send(foo)
# > NoMethodError: undefined method `foo' for #<Order...

For those who fears using send:

o.association(relationship).scope

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