Combine different collection result into one in Mongo - mongodb

Question

Below is my query I want the result of shp_tx_survey_with_index and
for each loop collection shp_counties_with_index name1 and name2 together of both this collection. If running this query separate then getting the result but this query gives me nothing. I want result like Range_Township, Survey, Section, abstract, centroid, name_1, name_2.
db.shp_tx_survey_with_index.aggregate(
[
{ $match: { "centroid": { "$ne": null } } },
{ $limit: 5 },
{
$project: {
Range_Township: "$l1surnam",
Survey: "$l4surnam",
Section: "$l1surnam",
abstract: "$abstract_",
centroid: "$centroid"
}
}
]
).forEach((obj) => {
var item = db.shp_counties_with_index.findOne({
geom_geojson: {
$nearSphere: {
$geometry: obj.centroid
}
}
}, { 'name_1': 1, 'name_2': 1 });
});
shp_counties_with_index sample collection
{
"_id" : ObjectId("5846bf55834d5b761f00000a"),
"engtype_2" : "County",
"geom_geojson" : {
"type" : "MultiPolygon",
"coordinates" : [
[
[
[
-73.6516685561232,
34.2445059658098
],
[
-73.6516685623318,
34.2445059757618
],
[
-73.6516685538257,
34.244505973301
],
[
-73.6516685561232,
34.2445059658098
]
]
] ]
},
"name_0" : "United States",
"name_1" : "Michigan",
"name_2" : "Chippewa",
"shape_area" : "0.481851809544",
"shape_leng" : "9.37720288177",
"type_2" : "County",
"validfr_2" : "Unknown",
"validto_2" : "Unknown",
"centroid" : {
"coordinates" : [
-73.65166855807875,
34.244505970785795
],
"type" : "Point"
}
}
shp_tx_survey_with_index sample collection
{
"_id" : ObjectId("5846bf76834d5b761f013fa7"),
"abstract_" : "321.000000000",
"abstract_i" : "322.000000000",
"anum" : "443962",
"area" : "0.0000666764235294",
"geom" : "01060000000100000001030000000100000008000000EC90DE47A07659C0F062332AEA813E403471FBB0A17759C06082096CE6813E4034A2C2ABA17759C0700AAF2731823E40B49BADAAA17759C09092F09440823E401C588E90A17759C000B4279A6A823E400019834C677559C02026721261823E403073564B677559C080C77880E6813E40EC90DE47A07659C0F062332AEA813E40",
"geom_geojson" : {
"type" : "MultiPolygon",
"coordinates" : [
[
[
[
-73.6517272344497,
34.2444627902475
],
[
-73.6517271719931,
34.2444627964974
],
[
-73.6517271718375,
34.2444627914072
],
[
-73.6517272344497,
34.2444627902475
]
]
]
]
},
"geom_text" : "MULTIPOLYGON(((-73.6517272344497 34.2444627902475,-73.6517271719931 34.2444627964974,-73.6517271718375 34.2444627914072,-73.6517272344497 34.2444627902475)))",
"gid" : "271508",
"l1surnam" : "TEMPLETON, J S",
"l2block" : null,
"l3surnum" : "4",
"l4surnam" : null,
"perimeter" : "0.0735082380545",
"probflag" : "0",
"shape_area" : "0.0000666764230571",
"shape_leng" : "0.0735082374282",
"centroid" : {
"coordinates" : [
-73.6517272031436,
34.24446279337245
],
"type" : "Point"
}
}
Thanks in advance.

Answer 1

When you want to combine information from 2 collections in a aggregation pipeline you can use the $lookup operator.
This operator is available from MongoDB 3.2 and up.