Mongodb group and push with empty arrays - mongodb

Question

I'm having a problem with a group when there is an array that could be empty.
The collection could be like this:
{
"_id" : "Contract_1",
"ContactId" : "Contact_1",
"Specifications" : [ ]
}
{
"_id" : "Contract_2",
"ContactId" : "Contact_2",
"Specifications" : [
{
"Description" : "Descrizione1",
"VehicleId" : "Vehicle_1",
"Customizations" : [
{
"Description" : "Random furniture",
"ContactId" : "Contact_5"
},
{
"Description" : "Random furniture 2",
"ContactId" : "Contact_3"
}
]
},
{
"Description" : "Descrizione2",
"VehicleId" : "Vehicle_2",
"Customizations" : [
{
"Description" : "Random furniture",
"ContactId" : "Contact_5"
},
{
"Description" : "Random furniture 2",
"ContactId" : "Contact_3"
}
]
}
]
}
{
"_id" : "Contract_3",
"ContactId" : "Contact_25",
"Specifications" : [
{
"Description" : "Descrizione1",
"VehicleId" : "Vehicle_1",
"Customizations" : []
},
{
"Description" : "Descrizione2",
"VehicleId" : "Vehicle_2",
"Customizations" : []
}
]
}
As you can see, sometimes Specifications can be null, and also Customizations.
And this is the query that I execute:
db.getCollection("Contract").aggregate([
{ "$lookup": {
"from": "Contact",
"localField": "ContactId",
"foreignField": "_id",
"as": "Contact"
}},
{ "$unwind": {"path":"$Contact", "preserveNullAndEmptyArrays":true }},
{ "$unwind": { "path": "$Specifications", "preserveNullAndEmptyArrays":true }},
{ "$lookup": {
"from": "Vehicle",
"localField": "Specifications.VehicleId",
"foreignField": "_id",
"as": "Specifications.Vehicle"
}},
{ "$unwind": {"path": {"$Specifications.Vehicle","preserveNullAndEmptyArrays":true} },
{ "$unwind": {"path": {"$Specifications.Customizations","preserveNullAndEmptyArrays":true} },
{ "$lookup": {
"from": "Contact",
"localField": "Specifications.Customizations.ContactId",
"foreignField": "_id",
"as": "Specifications.Customizations.Contact"
}},
{ "$unwind": {"path": {"$Specifications.Customizations.Contact","preserveNullAndEmptyArrays":true} },
{ "$group": {
"_id": {
"_id": "$_id",
"Description": "$Specifications.Description"
},
"ContactId": { "$first": "$ContactId" },
"Contact": { "$first": "$Contact" },
"Specifications": {
"$push": "$Specifications.Customizations"
}
}},
{ "$group": {
"_id": "$_id._id",
"ContactId": { "$first": "$ContactId" },
"Contact": { "$first": "$Contact" },
"Specifications": {
"$push": {
"Description": "$_id.Description",
"Customizations": "$Specifications"
}
}
}}
])
}},
{ "$group": {
"_id": "$_id._id",
"ContactId": { "$first": "$ContactId" },
"Contact": { "$first": "$Contact" },
"Specifications": {
"$push": {
"Description": "$_id.Description",
"Customizations": "$Specifications"
}
}
}}
])
Once the query execute, when it's doing the 2 $group it creates a problem, since for the first one when pushing $Specifications.Customizations will create an array with an empty element inside. What I want is that If Specifications is an empty array, will stay so without adding an empty element inside.

Answer 1

This is I can see one of the drawback of the $unwind and $group for the nested arrays. To get rid from this you need to add one more stage $addFields to filter out the empty nested arrays.
Add this at the end of the pipeline
{ "$addFields": {
"Specifications": {
"$filter": {
"input": "$Specifications",
"cond": { "$ne": ["$$this.Description", undefined] }
}
}
}}

Answer 2

For anyone still having the same problem like me and for whom the answer from #Ashh isnt working (I cant figure out the reason why it doesnt work for me). $ifNull instead of $ne is what worked for me, like this:
{ "$addFields": {
"Specifications": {
"$filter": {
"input": "$Specifications",
"cond": { "$ifNull": ["$$this.Description", false] }
}
}
}}