Querying Arrays

• Assume the following slides operate on a collection with the following documents

db.inventory.insertMany( [
   { item: "journal", instock: [ { warehouse: "A", qty: 5 }, { warehouse: "C", qty: 15 } ] },
   { item: "notebook", instock: [ { warehouse: "C", qty: 5 } ] },
   { item: "paper", instock: [ { warehouse: "A", qty: 60 }, { warehouse: "B", qty: 15 } ] },
   { item: "planner", instock: [ { warehouse: "A", qty: 40 }, { warehouse: "B", qty: 5 } ] },
   { item: "postcard", instock: [ { warehouse: "B", qty: 15 }, { warehouse: "C", qty: 35 } ] }
]);

Matching Nested Array Documents

• At least ONE of the documents in the nested array must match exactly
• Order of the properties matters in the query

db.inventory.find(
  { "instock":
    { warehouse: "A", qty: 5 }
  }
)

// Matching document
{ item: "journal", instock: [ { warehouse: "A", qty: 5 }, { warehouse: "C", qty: 15 } ] }

Querying Conditions on Nested Arrays 1

• To query with relaxed conditions, use the property name "." sub-document property concatenation within quotes.

db.inventory.find( { 'instock.qty': 5 } )

• Range conditions are also supported

db.inventory.find(
  { 'instock.qty':
    { $lte: 20 }
  }
)

Querying Conditions on Nested Arrays 2

• Multiple range conditions are also supported
• Is there a difference betweeen the two below queries?

db.inventory.find(
  { 'instock.qty':
    { $lte: 20, $gte: 5 }
  }
)

db.inventory.find(
  { 'instock.qty':
    { $lte: 20 },
    'instock.qty': { $gte: 5}
  }
)

Querying Array Documents at Index

• It is possible to direct the query to a specific Array document
• Syntax is: '<property>.<index>.<subProperty>'

db.inventory.find(
  { 'instock.0.qty':
    { $lte: 20 }
  }
)

Liberal Matching Without $elemMatch

• Warning: If you provide multiple conditions without $elemMatch the semantics mean: "If ANY COMBINATION of these conditions are met, then match"
• This makes the conditions act like OR condtions

db.inventory.find(
  {
    "instock.qty": 5,
    "instock.warehouse": "A"
  }
)

Querying with Many Conditions on Arrays

• $elemMatch matches any document with a sub-document that satisfies the full query
• This acts like an AND between the two conditions

db.inventory.find(
  { "instock":
    { $elemMatch:
      {
        qty: 5,
        warehouse: "A"
      }
    }
  }
)

Querying with Range Conditions on Arrays

• $elemMatch can be used with range conditions

db.inventory.find(
  { "instock":
    { $elemMatch:
      {
        qty: { $gte: 5 },
        warehouse: "A"
      }
    }
  }
)

Querying with Many Range Conditions on Arrays

• $elemMatch can also be used with multiple range conditions

db.inventory.find(
  { "instock":
    {
      $elemMatch: {
        qty: {
          $gte: 5,
          $lte: 20
        },
        warehouse: "A"
      }
    }
  }
)

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