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$eq

New in version 3.0.

Specifies equality condition. The $eq operator matches documents where the value of a field equals the specified value.

{ <field>: { $eq: <value> } }

Specifying the $eq operator is equivalent to using the form { field: <value> } except when the <value> is a regular expression. See below for examples.

Behavior

Comparison Order

For comparison of different BSON type values, see the specified BSON comparison order.

Match a Document Value

If the specified <value> is a document, the order of the fields in the document matters.

Match an Array Value

If the specified <value> is an array, MongoDB matches documents where the <field> matches the array exactly or the <field> contains an element that matches the array exactly. The order of the elements matters. For an example, see Equals an Array Value.

Match a Regular Expression

The expression { field: <value> } implicitly specifies a match on <value>. MongoDB translates the implicit match to a more explicit form.

When the <value> is fixed, like a particular string, the expression is equivalent to using the $eq operator { field: { $eq: <value> } }.

If <value> is a regular expression, the statement is expanded using the $regex operator { field: { $regex: <value> } }.

For examples illustrating this behaviour, see Regex Match Behaviour.

Security Implications

Always use the explicit form { field: { $eq: <value> } } with user-supplied input to avoid problems with maliciously formed queries.

Examples

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" ] }

Equals a Specified Value

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" ] }

Field in Embedded Document Equals a Value

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" ] }

Array Element Equals a Value

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" ] }
[1]The query will also match documents where the value of the tags field is the string "B".

Equals an Array Value

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" ] }

Regex Match Behaviour

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 string

A 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 expression

An 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 matches

A 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" }