evaluation-operators.md

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Evaluation query operators

Evaluation query operators return data based on the evaluation of a specified expression.

Operator	Description
$mod	Matches documents where the field element is divided by a given value and returns a the specified remainder value
$regex	Matches documents where a field matches a specified regular expression query

For the examples in this section, insert the following documents into the catalog collection:

db.catalog.insertMany([
  {
    product: 'bottle',
    price: 15,
    stock: 1
  },
  {
    product: 'spoon',
    price: 500,
    stock: 0
  },
  {
    product: 'cup',
    price: 100,
    stock: 14
  },
  {
    product: 'BoWL',
    price: 56,
    stock: 5
  },
  {
    product: 'boTtLe',
    price: 20,
    stock: 3
  }
])

$mod

Syntax: { <field>: { $mod: [ <divisor-value>, <modulus> ] } }

The $mod operator matches documents where the field element divided by a given value returns a specified remainder (otherwise known as a modulus). The mathematical operation for this is field-value % divisor-value = modulus.

Example: The following query returns all the documents where the value of the "stock" field is evenly divisible by 2:

db.catalog.find({
  stock: {
    $mod: [2, 0]
  }
})

The output:

[
  {
    _id: ObjectId("63e3ac0184f488929a3f737a"),
    product: 'spoon',
    price: 500,
    stock: 0
  },
  {
    _id: ObjectId("63e3ac0184f488929a3f737b"),
    product: 'cup',
    price: 100,
    stock: 14
  }
]

:::caution Note that the $mod expression returns an error if you only have a single element in the array, more than two elements in the array, or if the array is empty. It also rounds down decimal input down to zero (e.g. $mod: [ 3.5 , 2 ] is executed as $mod: [ 3 , 2 ]). :::

$regex

Syntax: { <field>: { $regex: '<expression-string>', $options: '<flag>' } }

Other syntaxes: { <field>: { $regex: /<expression-string>/, $options: '<flag>' } } and { <field>: /<expression-string>/<flag> }.

To use regular expression for queries on particular fields, use the $regex operator.

Example: The following query returns all the documents where the value of the "product" field starts with the letter "b":

db.catalog.find({
  product: {
    $regex: /^b/
  }
})

The output:

[
  {
    _id: ObjectId("63e4ce469695494b86bf2b2d"),
    product: 'bottle',
    price: 15,
    stock: 1
  },
  {
    _id: ObjectId("63e4ce469695494b86bf2b31"),
    product: 'boTtLe',
    price: 20,
    stock: 3
  }
]

$options is an optional parameter that specifies the regular expression flags to use, such as:

• Case-insensitivity (i)
• Multi-line matching (m)
• Dot character matching (s)

:::note The regex flag for ignoring white spaces (x) is not currently supported. Follow here for more updates. :::

To perform case-insensitive matching, use the i flag in the regex expression.

Example: The following query returns all the documents where the value of the "product" field is equal to "bottle" (case-insensitive):

db.catalog.find({
  product: {
    $regex: /bottle/i
  }
})

The output:

[
  {
    _id: ObjectId("63e3ac0184f488929a3f7379"),
    product: 'bottle',
    price: 15,
    stock: 1
  },
  {
    _id: ObjectId("63e3ac0184f488929a3f737d"),
    product: 'boTtLe',
    price: 20,
    stock: 3
  }
]