$ python mongo.py
Query with Expression
>>> db.inventory.find('status == "D"')  # the default db is "test"
query: {'$expr': {'$eq': ['$status', 'D']}}
{'_id': ObjectId('5b5a45738507c4e0b7f2bcd8'), 'item': 'paper', 'qty': 100.0, 'size': {'h': 8.5, 'w': 11.0, 'uom': 'in'}, 'status': 'D'}
...

>>> db = client.market  # change db to "market"

>>> db.items.find({})
query: {}
{'_id': ObjectId('5b9164ff73556ec902b1e269'), 'name': 'iPhone 7', 'type': 'phone'}

>>> db = client.test  # go back to test DB

>>> db.inventory.find('status == "A" and qty < 30')
query: {'$expr': {'$and': [{'$eq': ['$status', 'A']}, {'$lt': ['$qty', 30]}]}}
{'_id': ObjectId('5b5a45738507c4e0b7f2bcd6'), 'item': 'journal', 'qty': 25.0, 'size': {'h': 14.0, 'w': 21.0, 'uom': 'cm'}, 'status': 'A'}

>>> db.inventory.find('status == "A" or qty < 30')
query: {'$expr': {'$or': [{'$eq': ['$status', 'A']}, {'$lt': ['$qty', 30]}]}}
{'_id': ObjectId('5b5a45738507c4e0b7f2bcd6'), 'item': 'journal', 'qty': 25.0, 'size': {'h': 14.0, 'w': 21.0, 'uom': 'cm'}, 'status': 'A'}
...

>>> db.inventory.find('(status == "A") and (qty < 30 or item == "paper")')
query: {'$expr': {'$and': [{'$eq': ['$status', 'A']}, {'$or': [{'$lt': ['$qty', 30]}, {'$eq': ['$item', 'paper']}]}]}}
{'_id': ObjectId('5b5a45738507c4e0b7f2bcd6'), 'item': 'journal', 'qty': 25.0, 'size': {'h': 14.0, 'w': 21.0, 'uom': 'cm'}, 'status': 'A'}

>>> db.inventory.find('size == { h: 14, w: 21, uom: "cm" }')
query: {'$expr': {'$eq': ['$size', {'h': 14, 'w': 21, 'uom': 'cm'}]}}
{'_id': ObjectId('5b5a45738507c4e0b7f2bcd6'), 'item': 'journal', 'qty': 25.0, 'size': {'h': 14.0, 'w': 21.0, 'uom': 'cm'}, 'status': 'A'}

>>> db.inventory.find('size.uom == "in"')
query: {'$expr': {'$eq': ['$size.uom', 'in']}}
{'_id': ObjectId('5b5a45738507c4e0b7f2bcd7'), 'item': 'notebook', 'qty': 50.0, 'size': {'h': 8.5, 'w': 11.0, 'uom': 'in'}, 'status': 'A'}
...

>>> db.inventory.find('size.h < 15 ')
query: {'$expr': {'$lt': ['$size.h', 15]}}
{'_id': ObjectId('5b5a45738507c4e0b7f2bcd6'), 'item': 'journal', 'qty': 25.0, 'size': {'h': 14.0, 'w': 21.0, 'uom': 'cm'}, 'status': 'A'}
...

# arrays
>>> db.inventory2.find( 'tags == ["red", "blank"]' )
query: {'$expr': {'$eq': ['$tags', ['red', 'blank']]}}
{'_id': ObjectId('5b5f7cf05d8d5e3f5d167119'), 'item': 'notebook', 'qty': 50.0, 'tags': ['red', 'blank'], 'dim_cm': [14.0, 21.0]}
...

>>> db.inventory2.find('"red" in tags')
query: {'$expr': {'$in': ['red', '$tags']}}
{'_id': ObjectId('5b5f7cf05d8d5e3f5d167118'), 'item': 'journal', 'qty': 25.0, 'tags': ['blank', 'red'], 'dim_cm': [14.0, 21.0]}
...

>>> db.inventory2.find('dim_cm[1] > 25')
query: {'$expr': {'$gt': [{'$arrayElemAt': ['$dim_cm', 1]}, 25]}}
{'_id': ObjectId('5b5f7cf05d8d5e3f5d16711b'), 'item': 'planner', 'qty': 75.0, 'tags': ['blank', 'red'], 'dim_cm': [22.85, 30.0]}
...

>>> db.inventory2.find('len(tags) == 3')
query: {'$expr': {'$eq': [{'$size': '$tags'}, 3]}}
{'_id': ObjectId('5b5f7cf05d8d5e3f5d16711a'), 'item': 'paper', 'qty': 100.0, 'tags': ['red', 'blank', 'plain'], 'dim_cm': [14.0, 21.0]}

>>> db.inventory2.find('len(tags) < 3')
query: {'$expr': {'$lt': [{'$size': '$tags'}, 3]}}
{'_id': ObjectId('5b5f7cf05d8d5e3f5d167118'), 'item': 'journal', 'qty': 25.0, 'tags': ['blank', 'red'], 'dim_cm': [14.0, 21.0]}
...

>>> db.inventory3.find('item == None')
query: {'$expr': {'$eq': ['$item', None]}}

>>> db.inventory.find('item == missing')
query: {'$expr': {'$eq': ['$item', '$missing']}}

>>> db.inventory2.find('len(filter(lambda x: x < 20 or x > 15, dim_cm)) > 0')
query: {'$expr': {'$gt': [{'$size': {'$filter': {'input': '$dim_cm', 'as': 'x', 'cond': {'$or': [{'$lt': ['$$x', 20]}, {'$gt': ['$$x', 15]}]}}}}, 0]}}