Unable to find a match in MongoDB query - database

Question

> db.inventory.find( )
{ "_id" : ObjectId("5eb67598bee5213484d45087"), "item" : "journal", "qty" : 25, "status" : "A", "size" : { "h" : 14, "w" : 21, "uom" : "cm" }, "tags" : [ "blank", "red" ] }
{ "_id" : ObjectId("5eb67598bee5213484d45088"), "item" : "notebook", "qty" : 50, "status" : "A", "size" : { "h" : 8.5, "w" : 11, "uom" : "in" }, "tags" : [ "red", "blank" ] }
{ "_id" : ObjectId("5eb67598bee5213484d45089"), "item" : "paper", "qty" : 10, "status" : "D", "size" : { "h" : 8.5, "w" : 11, "uom" : "in" }, "tags" : [ "red", "blank", "plain" ] }
{ "_id" : ObjectId("5eb67598bee5213484d4508a"), "item" : "planner", "qty" : 0, "status" : "D", "size" : { "h" : 22.85, "w" : 30, "uom" : "cm" }, "tags" : [ "blank", "red" ] }
{ "_id" : ObjectId("5eb67598bee5213484d4508b"), "item" : "postcard", "qty" : 45, "status" : "A", "size" : { "h" : 10, "w" : 15.25, "uom" : "cm" }, "tags" : [ "blue" ] }
{ "_id" : ObjectId("5ebfd02b3a3b38a52be04608"), "item" : "journal", "qty" : 25, "tags" : [ "blank", "red" ], "dim_cm" : [ 14, 21 ] }
{ "_id" : ObjectId("5ebfd02b3a3b38a52be04609"), "item" : "notebook", "qty" : 50, "tags" : [ "red", "blank" ], "dim_cm" : [ 14, 21 ] }
{ "_id" : ObjectId("5ebfd02b3a3b38a52be0460a"), "item" : "paper", "qty" : 100, "tags" : [ "red", "blank", "plain" ], "dim_cm" : [ 14, 21 ] }
{ "_id" : ObjectId("5ebfd02b3a3b38a52be0460b"), "item" : "planner", "qty" : 75, "tags" : [ "blank", "red" ], "dim_cm" : [ 22.85, 30 ] }
{ "_id" : ObjectId("5ebfd02b3a3b38a52be0460c"), "item" : "postcard", "qty" : 45, "tags" : [ "blue" ], "dim_cm" : [ 10, 15.25 ] }
>
> db.inventory.find( { item: 'journal', qty: '25' } )
>
item: 'journal' with qty '25' is the first item. Why isn't it being shown?
Here is a similar query which is working as per them: https://www.w3resource.com/mongodb-exercises/mongodb-exercise-22.php
db.restaurants.find(
{
"grades.date": ISODate("2014-08-11T00:00:00Z"),
"grades.grade":"A" ,
"grades.score" : 11
},
{"restaurant_id" : 1,"name":1,"grades":1}
);

Answer 1

qty is a number and you're trying to find a string, try:
db.inventory.find( { item: 'journal', qty: 25 } )
(docs)
For most data types, however, comparison operators only perform comparisons on documents where the BSON type of the target field matches the type of the query operand.