In this stage, you can group your output by the specified field. This field must be present in each item in your list.
This stage follows this pattern: { groupBy: <field> }.
Example
Consider the following data set:
[
{ author: 'John', title: 'abc' },
{ author: 'Donna', title: 'xyz' },
{ author: 'Donna', title: 'mnp' },
{ author: 'Donna', title: 'def' },
{ author: 'John', title: 'uvw' },
{ author: 'Gemma', title: 'opr' }
]{ groupBy: 'author' }will group all items by title, returning:
[
'John': [{ author: 'John', title: 'abc' }, { author: 'John', title: 'uvw' }],
'Donna': [{ author: 'Donna', title: 'xyz' }, { author: 'Donna', title: 'mnp' }, { author: 'Donna', title: 'def' }],
'Gemma': [{ author: 'Gemma', title: 'opr' }]
]