Last journal showed how we can use $project to include and exclude document fields. With $set, you can add new fields (e.g., out of concatenation) or modify existing fields.
Suppose we have these birds documents. We want to show a new field name with this format: common_name (scientific_name).
// db.birds.find({})
[
{
_id: ObjectId("62684d24613e55b82d7065d9"),
common_name: "Palm Warbler",
scientific_name: "Setaphaga palmarum",
wingspan_cm: 20.32,
habitat: "field",
diet: ["insects", "seeds", "berries"],
last_seen: ISODate("2022-05-19T20:20:44.083Z"),
},
{
_id: ObjectId("626845c4613e55b82d7065d5"),
common_name: "Sanderling",
scientific_name: "Calidris alba",
wingspan_cm: 43.18,
habitat: "wetlands",
diet: ["crustaceans", "oceanic invertebrates"],
last_seen: ISODate("2022-05-19T20:20:44.083Z"),
},
{
_id: ObjectId("626844cc613e55b82d7065d3"),
common_name: "Bufflehead",
scientific_name: "Bucephala albeola",
wingspan_cm: 53.3,
habitat: "wetlands",
diet: ["snails", "crustaceans", "water plants"],
last_seen: ISODate("2022-05-19T20:20:44.083Z"),
},
{
_id: ObjectId("62684641613e55b82d7065d6"),
common_name: "Rock Pigeon",
scientific_name: "Columba livia",
wingspan_cm: 71.12,
habitat: ["urban parks", "farmland"],
diet: ["grass", "grain", "fruit", "weeds", "vegetables"],
last_seen: ISODate("2022-05-19T20:20:44.083Z"),
},
{
_id: ObjectId("6268413c613e55b82d7065d2"),
common_name: "Canada Goose",
scientific_name: "Branta canadensis",
wingspan_cm: 152.4,
habitat: "wetlands",
diet: ["grass", "algae"],
last_seen: ISODate("2022-05-19T20:20:44.083Z"),
},
{
_id: ObjectId("6268471e613e55b82d7065d7"),
common_name: "Great Horned Owl",
scientific_name: "Bubo virginianus",
wingspan_cm: 111.76,
habitat: ["grasslands", "farmland", "tall forest"],
diet: ["mice", "small mammals", "rabbits", "shrews"],
last_seen: ISODate("2022-05-19T20:20:44.083Z"),
},
{
_id: ObjectId("6286a5612f3fa87b7d86dcd2"),
common_name: "Grackle",
scientific_name: "Quiscalus quiscula",
wingspan_cm: 28.04,
habitat: "pine trees",
diet: ["insects", "minnows", "eggs"],
sightings_count: 7,
last_seen: ISODate("2022-05-19T20:20:44.083Z"),
},
{
_id: ObjectId("62cddf53c1d62bc45439bebf"),
common_name: "Grey Catbird",
scientific_name: "Dumetella carolinensis",
wingspan_cm: 26.04,
habitat: ["vegetation", "Scrublands", "woodlands"],
diet: ["fruit", "berries", "worms"],
sightings_count: 3,
},
{
_id: ObjectId("62684c8c613e55b82d7065d8"),
common_name: "Purple Martin",
scientific_name: "Progne Subis",
wingspan_cm: 45.72,
habitat: "field",
diet: ["insects"],
last_seen: ISODate("2022-05-19T20:20:44.083Z"),
},
{
_id: ObjectId("628682d92f3fa87b7d86dcce"),
common_name: "Blue Jay",
scientific_name: "Cyanocitta cristata",
wingspan_cm: 34.17,
habitat: "forests",
diet: ["vegetables", "nuts", "grains"],
sightings_count: 4,
last_seen: ISODate("2022-05-19T20:20:44.083Z"),
},
{
_id: ObjectId("62684551613e55b82d7065d4"),
common_name: "Great Blue Heron",
scientific_name: "Ardea herodias",
wingspan_cm: 182.88,
habitat: "wetlands",
diet: ["snails", "crustaceans", "fish", "frogs", "snakes"],
last_seen: ISODate("2022-05-19T20:20:44.083Z"),
},
{
_id: ObjectId("6286809e2f3fa87b7d86dccd"),
common_name: "Northern Cardinal",
scientific_name: "Cardinalis cardinalis",
wingspan_cm: 25.32,
habitat: "woodlands",
diet: ["grain", "seeds", "fruit"],
last_seen: ISODate("2022-05-19T20:20:44.083Z"),
},
];To combine field values, we can use the aggregation operator $concat.
// db.birds.aggregate([{$project: {common_name: 1, scientific_name: 1, _id: 0}}, {$set: {name: {$concat: ["$common_name", " (", "$scientific_name", ")"]}}}])
[
{
common_name: "Palm Warbler",
scientific_name: "Setaphaga palmarum",
name: "Palm Warbler (Setaphaga palmarum)",
},
{
common_name: "Sanderling",
scientific_name: "Calidris alba",
name: "Sanderling (Calidris alba)",
},
{
common_name: "Bufflehead",
scientific_name: "Bucephala albeola",
name: "Bufflehead (Bucephala albeola)",
},
{
common_name: "Rock Pigeon",
scientific_name: "Columba livia",
name: "Rock Pigeon (Columba livia)",
},
{
common_name: "Canada Goose",
scientific_name: "Branta canadensis",
name: "Canada Goose (Branta canadensis)",
},
{
common_name: "Great Horned Owl",
scientific_name: "Bubo virginianus",
name: "Great Horned Owl (Bubo virginianus)",
},
{
common_name: "Grackle",
scientific_name: "Quiscalus quiscula",
name: "Grackle (Quiscalus quiscula)",
},
{
common_name: "Grey Catbird",
scientific_name: "Dumetella carolinensis",
name: "Grey Catbird (Dumetella carolinensis)",
},
{
common_name: "Purple Martin",
scientific_name: "Progne Subis",
name: "Purple Martin (Progne Subis)",
},
{
common_name: "Blue Jay",
scientific_name: "Cyanocitta cristata",
name: "Blue Jay (Cyanocitta cristata)",
},
{
common_name: "Great Blue Heron",
scientific_name: "Ardea herodias",
name: "Great Blue Heron (Ardea herodias)",
},
{
common_name: "Northern Cardinal",
scientific_name: "Cardinalis cardinalis",
name: "Northern Cardinal (Cardinalis cardinalis)",
},
];