Match ID and fetch documents with $eq in MongoDB in case of array?

Use the $eq operator along with find() to match ID values within arrays and fetch documents. The $eq specifies an equality condition and matches documents where any array element equals the specified value.

Syntax

db.collection.find({"arrayField": {$eq: "value"}});

// Alternative (equivalent) syntax
db.collection.find({"arrayField": "value"});

Sample Data

Let us create a collection with documents ?

db.demo426.insertMany([
    {"Ids": ["110", "120", "101"]},
    {"Ids": ["100", "201", "401"]},
    {"Ids": ["501", "600", "700"]}
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5e75e50fbbc41e36cc3cae72"),
        ObjectId("5e75e51abbc41e36cc3cae73"),
        ObjectId("5e75e527bbc41e36cc3cae74")
    ]
}

Display all documents from the collection ?

db.demo426.find().pretty();

{
    "_id": ObjectId("5e75e50fbbc41e36cc3cae72"),
    "Ids": [
        "110",
        "120", 
        "101"
    ]
}
{
    "_id": ObjectId("5e75e51abbc41e36cc3cae73"),
    "Ids": [
        "100",
        "201",
        "401"
    ]
}
{
    "_id": ObjectId("5e75e527bbc41e36cc3cae74"),
    "Ids": [
        "501",
        "600",
        "700"
    ]
}

Example: Match ID with $eq

Find documents where the Ids array contains the value "501" ?

db.demo426.find({"Ids": {$eq: "501"}});

{ "_id": ObjectId("5e75e527bbc41e36cc3cae74"), "Ids": ["501", "600", "700"] }

Key Points

• When using $eq with arrays, MongoDB checks if any element in the array matches the specified value.
• For array queries, {"field": {$eq: "value"}} and {"field": "value"} are equivalent.
• The query returns the entire document if any array element matches.

Conclusion

The $eq operator effectively matches individual elements within arrays. It returns documents where any array element equals the specified value, making it useful for searching collections with array fields.