pattern attribute

Use Attribute Pattern for Sparse or Variable Fields

If documents have many optional fields, move them into a key-value array. This avoids dozens of sparse indexes and lets you query across attributes with a single multikey index.

Incorrect (separate field and index per optional attribute):

// Many optional fields - most are missing on any given document
{
  _id: 1,
  name: "Bottle",
  color: "red",
  size: "M",
  material: "glass",
  // 20+ other optional fields, varying per document
}

// One partial index per optional field — correct use of partialFilterExpression,
// but you end up maintaining dozens of indexes as attributes grow
db.items.createIndex({ color: 1 }, { partialFilterExpression: { color: { $exists: true } } })
db.items.createIndex({ size: 1 }, { partialFilterExpression: { size: { $exists: true } } })
db.items.createIndex({ material: 1 }, { partialFilterExpression: { material: { $exists: true } } })
// … repeated for every new attribute

Correct (attribute pattern):

// Store optional fields as key-value pairs
{
  _id: 1,
  name: "Bottle",
  attributes: [
    { k: "color", v: "red" },
    { k: "size", v: "M" },
    { k: "material", v: "glass" }
  ]
}

// Single multikey index for all attributes

db.items.createIndex({ "attributes.k": 1, "attributes.v": 1 })

// Query for color = red

db.items.find({
  attributes: { $elemMatch: { k: "color", v: "red" } }
})

When NOT to use this pattern:

• Fixed schema: If fields are stable and always present.
• Type-specific validation: If each field needs strict schema rules.
• Single-field queries only: A normal field may be simpler and faster.
• Atlas Search workloads: The { k, v } key-value structure cannot be mapped as named fields in Atlas Search indexes. If you need full-text search on attribute values by key name, use static named fields instead.

Verify with

// Ensure queries use the multikey index

db.items.find({
  attributes: { $elemMatch: { k: "material", v: "glass" } }
}).explain("executionStats")

Reference: Attribute Pattern