mongodb aggregation - setting the contents of array as input, while storing sibling keys - mongodb-query

Question

I'm in the middle of an aggregation pipeline when I have the following data passed from the previous step as input to a "count all leaves in all branches/trees/forests" as indicated in this question: Need guidance on mongo aggregate lookup to count subnodes in all child nodes for each node
{
forest_ranger: "bob",
some_other_data: {},
forests: [
{
forest_id: 'forestA',
trees: [
{
tree_id: 'treeA',
branches: [
{
branch_id: 'branchA',
}
]
}
]
}
],
}
I have that all working properly, but only if I'm reading directly from a collection.
My question is - how can I
a) save forest_ranger and some_other_data "for later"
b) and start the next stage by mapping forests as the array of
documents that are input to the "count all leaves"?
Note - I tried to get just (b) working for starters, but I can't seem to treat "forests" as an array so that the solution in the link above can do its magic.
Then- when that phase is done, extract what was stored in (a) to get the original format back with all the counts.

Answer 1

That is the main disadvantage of using $group. You have to explicitly carry forward every non participating fields using either $first or $last like this:
Get the test data with those additional fields from this new MongoPlayground link.
Check the updated query taken from the link:
db.Forests.aggregate([
{ $unwind: "$forests" },
{ $unwind: "$forests.trees" },
{ $unwind: "$forests.trees.branches" },
{
$lookup: {
from: "Leaves",
localField: "forests.trees.branches.branch_id",
foreignField: "branch_id",
as: "forests.trees.branches.leaves"
}
},
{
$addFields: {
"forests.trees.branches.leaf_count": { $size: "$forests.trees.branches.leaves" }
}
},
{
$project: { "forests.trees.branches.leaves": 0 }
},
{
$group: {
_id: {
forest_id: "$forests.forest_id",
tree_id: "$forests.trees.tree_id"
},
forest_ranger: { $first: "$forest_ranger" },
some_other_data: { $first: "$some_other_data" },
leaf_count: { $sum: "$forests.trees.branches.leaf_count" },
branches: { $push: "$forests.trees.branches" }
}
},
{
$group: {
_id: "$_id.forest_id",
forest_ranger: { $first: "$forest_ranger" },
some_other_data: { $first: "$some_other_data" },
leaf_count: { $sum: "$leaf_count" },
trees: {
$push: {
leaf_count: { $sum: "$leaf_count" },
tree_id: "$_id.tree_id",
branches: "$branches"
}
}
}
},
{
$group: {
_id: null,
forest_ranger: { $first: "$forest_ranger" },
some_other_data: { $first: "$some_other_data" },
leaf_count: { $sum: "$leaf_count" },
forests: {
$push: {
leaf_count: { $sum: "$leaf_count" },
forest_id: "$_id",
trees: "$trees"
}
}
}
},
{
$project: { _id: 0 }
}
])
Output:
{
"forest_ranger" : "bob",
"some_other_data" : {
"data" : "Dummy data"
},
"leaf_count" : 4,
"forests" : [
{
"leaf_count" : 3,
"forest_id" : "forestA",
"trees" : [
{
"leaf_count" : 1,
"tree_id" : "treeB",
"branches" : [
{
"branch_id" : "branchB",
"leaf_count" : 1
}
]
},
{
"leaf_count" : 2,
"tree_id" : "treeA",
"branches" : [
{
"branch_id" : "branchA",
"leaf_count" : 1
},
{
"branch_id" : "branchA1",
"leaf_count" : 1
},
{
"branch_id" : "branchA2",
"leaf_count" : 0
}
]
}
]
},
{
"leaf_count" : 1,
"forest_id" : "forestB",
"trees" : [
{
"leaf_count" : 0,
"tree_id" : "treeD",
"branches" : [
{
"branch_id" : "branchD",
"leaf_count" : 0
}
]
},
{
"leaf_count" : 1,
"tree_id" : "treeC",
"branches" : [
{
"branch_id" : "branchC",
"leaf_count" : 1
}
]
}
]
},
{
"leaf_count" : 0,
"forest_id" : "forestC",
"trees" : [
{
"leaf_count" : 0,
"tree_id" : "treeE",
"branches" : [
{
"branch_id" : "branchE",
"leaf_count" : 0
}
]
}
]
}
]
}