db.createView() by matching and comparing two tables in mongodb - mongodb

Question

I would like to createView by comparing and grouping two table in mongodb.
table1
{id: 1, item: 'apple', house: 'A'},
{id: 2, item: 'apple', house: 'A'},
{id: 3, item: 'apple', house: 'B'},
{id: 4, item: 'banana', house: 'B'},
{id: 5, item: 'banana', house: 'C'},
{id: 6, item: 'pear', house: 'A'},
{id: 7, item: 'pear', house: 'B'},
{id: 8, item: 'pear', house: 'A'},
{id: 9, item: 'pear', house: 'C'},
And in table 2, I need to compare and match
table2
{id: 1, fruits: 'apple', type: 'important'},
{id: 2, fruits: 'banana', type: 'important'},
{id: 3, fruits: 'pear', type: 'notImportant'},
The result I want to get:
houses
{id: 1, house: 'A', totalItem: 4, noOfImportant: 2 },
{id: 2, house: 'B', totalItem: 2, noOfImportant: 2},
{id: 3, house: 'C', totalItem: 2, noOfImportant: 1},
I have tried:
db.createView(
'houses',
'table1',
[
{$lookup: { from: 'table2', localField: 'fruits', foreignField: 'item', as: 'fruits'}},
{
'$group': {
house: '$house',
totalItem: {$sum:1},
noOfImportant: {$sum:'$table2.notImpotant'},
}
},
]
)
but I can't seem to get any thing out. please help me. Thank you very much in advance!

Answer 1

The $lookup stage will return an array, even if it only finds a single matching document.
You can use the $arrayElemAt operator to pick out the first element, or you could simply $unwind the result.
Either way, it looks like there was a typo in your group: noOfImportant: {$sum:'$table2.notImpotant'}, - there doesn't appear to be a field named "table2", I suspect you meant to check the type field of the fruits document.
The $group stage requires that there be an explicitly defined _id field, if you need that field to be called house you can to that afterward with projection.
Since the type field is not numeric, you would need to use $cond in order to count them up.
db.createView(
'houses',
'table1',
[
{$lookup: {
from: 'table2',
localField: 'item',
foreignField: 'fruits',
as: 'fruits'
}},
{$unwind: {
path: '$fruits',
preserveNullAndEmptyArrays: true
}},
{$group: {
_id: '$house',
totalItem: {$sum:1},
noOfImportant: {
$sum:{
$cond:{
if:{$eq:['$fruits.type', 'important']},
then: 1,
else: 0
}
}
},
}},
{$addFields: { house: "$_id" }},
{$project: { _id: 0 }}
]
)
The aggregate functions returns much more descriptive error messages. You should test the pipeline prior to creating the view.
Here is a MongoPlayground that shows this pipeline in action.