ruby on rails - Weird results using distinct() and count() methods on apparently similar code -

the following strange behavior has been spotted rails 4 (rails 4.2.2/ruby 2.2.1) , postgresql 9 (postgresql 9.4.4).

keeping in mind grouping without aggregate expressions calculates set of distinct values in column, following apparently similar lines of code lead strange , weird results:

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why does:

item.select( :color, :size ).distinct.count  # error 

return error, while instead:

item.select( :color, :size ).distinct.count( :all )  # wrong item.select( :color, :size ).distinct.count( :color, :size )  # wrong 

both return wrong result (if there duplicate records same values of :color , :size)?

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and why does:

item.select( :color, :size ).group( :color, :size ).count  # error 

return error, while instead:

item.select( :color, :size ).group( :color, :size ).count( :all ).count 

correctly counts select count(distinct (color, size)) items?

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also:

item.group( :color, :size )  # error 

return error, expected because queries group by clause can include in select list non-aggregated column references if appears in, or functionally dependent upon, group by clause.

but instead rails allows following:

item.group( :color, :size ).count.count 

that correctly counts select count(distinct (color, size)) items!