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$eq
Specifies equality condition.
The $eq
operator matches documents where the value of a field equals the specified value.
{ : { $eq: } }
Specifying the $eq
operator is equivalent to using the form { field: }
except when the is a regular expression.
See below for examples.
For comparison of different BSON type values, see the specified BSON comparison order.
If the specified is a document, the order
of the fields in the document matters.
If the specified is an array, MongoDB matches documents where the
matches the array exactly or the
contains an element that matches the array exactly. The order of the elements matters. For an example, see Equals an Array Value.
The expression
{ field: }
implicitly specifies a match on . MongoDB translates the implicit match to a more explicit form.
When the is fixed, like a particular string, the expression is equivalent to using the
$eq
operator { field: { $eq: } }
.
If is a regular expression, the statement is expanded using the
$regex
operator { field: { $regex: } }
.
For examples illustrating this behaviour, see Regex Match Behaviour.
Always use the explicit form { field: { $eq: } }
with user-supplied input to avoid problems with maliciously formed queries.
The following examples query against the inventory
collection with the following documents:
{ _id: 1, item: { name: "ab", code: "123" }, qty: 15, tags: [ "A", "B", "C" ] } { _id: 2, item: { name: "cd", code: "123" }, qty: 20, tags: [ "B" ] } { _id: 3, item: { name: "ij", code: "456" }, qty: 25, tags: [ "A", "B" ] } { _id: 4, item: { name: "xy", code: "456" }, qty: 30, tags: [ "B", "A" ] } { _id: 5, item: { name: "mn", code: "000" }, qty: 20, tags: [ [ "A", "B" ], "C" ] }
The following example queries the inventory
collection to select all documents where the value of the qty
field equals 20
:
db.inventory.find[ { qty: { $eq: 20 } } ]
The query is equivalent to:
db.inventory.find[ { qty: 20 } ]
Both queries match the following documents:
{ _id: 2, item: { name: "cd", code: "123" }, qty: 20, tags: [ "B" ] } { _id: 5, item: { name: "mn", code: "000" }, qty: 20, tags: [ [ "A", "B" ], "C" ] }
The following example queries the inventory
collection to select all documents where the value of the name
field in the item
document equals "ab"
. To specify a condition on a field in an embedded document, use the
dot notation.
db.inventory.find[ { "item.name": { $eq: "ab" } } ]
The query is equivalent to:
db.inventory.find[ { "item.name": "ab" } ]
Both queries match the following document:
{ _id: 1, item: { name: "ab", code: "123" }, qty: 15, tags: [ "A", "B", "C" ] }
Tip
See also:
The following example queries the inventory
collection to select all documents where the tags
array contains an element with the value
"B"
[1]:
db.inventory.find[ { tags: { $eq: "B" } } ]
The query is equivalent to:
db.inventory.find[ { tags: "B" } ]
Both queries match the following documents:
{ _id: 1, item: { name: "ab", code: "123" }, qty: 15, tags: [ "A", "B", "C" ] } { _id: 2, item: { name: "cd", code: "123" }, qty: 20, tags: [ "B" ] } { _id: 3, item: { name: "ij", code: "456" }, qty: 25, tags: [ "A", "B" ] } { _id: 4, item: { name: "xy", code: "456" }, qty: 30, tags: [ "B", "A" ] }
Tip
The following example queries the inventory
collection to select all documents where the tags
array equals exactly
the specified array or the tags
array contains an element that equals the array [ "A", "B" ]
.
db.inventory.find[ { tags: { $eq: [ "A", "B" ] } } ]
The query is equivalent to:
db.inventory.find[ { tags: [ "A", "B" ] } ]
Both queries match the following documents:
{ _id: 3, item: { name: "ij", code: "456" }, qty: 25, tags: [ "A", "B" ] } { _id: 5, item: { name: "mn", code: "000" }, qty: 20, tags: [ [ "A", "B" ], "C" ] }
The following examples demonstrate the differences in behavior between implicit and explict regular expression matching. Consider a collection with these documents:
{ _id: 001, company: "MongoDB" } { _id: 002, company: "MongoDB2" }
$eq match on a stringA string expands
to return the same values whether an implicit match or an explicit use of $eq
. Both of these queries:
db.collection.find[ { company: "MongoDB" }, {_id: 0 }] db.collection.find[ { company: { $eq: "MongoDB" } }, {_id: 0 } ]
return the following result:
{ "company" : "MongoDB" }
$eq match on a regular expressionAn explicit query using $eq
and a regular expression will only match an object which is also a regular expresssion. The example query won't return anything since values in the company
field are strings.
db.collection.find[ { company: { $eq: /MongoDB/ } }, {_id: 0 } ]
Regular expression
matchesA query with an implicit match against a regular expression is equivalent to a making a query with the $regex
operator. Both of these queries:
db.collection.find[ { company: /MongoDB/ }, {_id: 0 }] db.collection.find[ { company: { $regex: /MongoDB/ } }, {_id: 0 } ]
return the same results:
{ "company" : "MongoDB" } { "company" : "MongoDB2" }