Useful Operators
{ $gt: [ <expression1>, <expression2> ] }
{ $lt: [ <expression1>, <expression2> ] }
{ $concat: [ <expression1>, <expression2>, ... ] }
{ $sum: <expression> }
{ $in: [ <expression>, <array expression> ] }
{ $max: <expression> }
{ $min: <expression> }
{ $avg: <expression> }
{ $divide: [ <expression1>, <expression2> ] }
{ $subtract: [ <expression1>, <expression2> ] }
{ $add: [ <expression1>, <expression2>, ... ] }
{ $push: <expression> }
{ $unwind: <field path> }
{ $concat: [ <expression1>, <expression2>, ... ] }
{ $cond: [ <boolean-expression>, <true-case>, <false-case> ] }