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array.group_by(&:item)
### or
array.group_by{|g| g.item}
## eg
array = [
{ date: '', mrr: '' },
{ date: '', mrr: '' },
{ date: '', mrr: '' },
{ date: '', mrr: '' },
{ date: '', mrr: '' }
]
array.group_by(&:date)
array.group_by{|g| g.date}