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Storing Embeddings in A Vector Database

After generating embeddings, you need to store them in a database that supports vector similarity search. Mastra provides a consistent interface for storing and querying embeddings across various vector databases.

Supported Databases​

vector-store.ts
import { MongoDBVector } from "@mastra/mongodb";

const store = new MongoDBVector({
  uri: process.env.MONGODB_URI,
  dbName: process.env.MONGODB_DATABASE,
});
await store.createIndex({
  indexName: "myCollection",
  dimension: 1536,
});
await store.upsert({
  indexName: "myCollection",
  vectors: embeddings,
  metadata: chunks.map((chunk) => ({ text: chunk.text })),
});

For detailed setup instructions and best practices, see the official MongoDB Atlas Vector Search documentation.

Using Vector Storage​

Once initialized, all vector stores share the same interface for creating indexes, upserting embeddings, and querying.

Creating Indexes​

Before storing embeddings, you need to create an index with the appropriate dimension size for your embedding model:

store-embeddings.ts
// Create an index with dimension 1536 (for text-embedding-3-small)
await store.createIndex({
  indexName: "myCollection",
  dimension: 1536,
});

The dimension size must match the output dimension of your chosen embedding model. Common dimension sizes are:

  • OpenAI text-embedding-3-small: 1536 dimensions (or custom, e.g., 256)
  • Cohere embed-multilingual-v3: 1024 dimensions
  • Google text-embedding-004: 768 dimensions (or custom)

Important: Index dimensions cannot be changed after creation. To use a different model, delete and recreate the index with the new dimension size.

Naming Rules for Databases​

Each vector database enforces specific naming conventions for indexes and collections to ensure compatibility and prevent conflicts.

Collection (index) names must:

  • Start with a letter or underscore
  • Be up to 120 bytes long
  • Contain only letters, numbers, underscores, or dots
  • Cannot contain $ or the null character
  • Example: my_collection.123 is valid
  • Example: my-index is not valid (contains hyphen)
  • Example: My$Collection is not valid (contains $)

Upserting Embeddings​

After creating an index, you can store embeddings along with their basic metadata:

store-embeddings.ts
// Store embeddings with their corresponding metadata
await store.upsert({
  indexName: "myCollection", // index name
  vectors: embeddings, // array of embedding vectors
  metadata: chunks.map((chunk) => ({
    text: chunk.text, // The original text content
    id: chunk.id, // Optional unique identifier
  })),
});

The upsert operation:

  • Takes an array of embedding vectors and their corresponding metadata
  • Updates existing vectors if they share the same ID
  • Creates new vectors if they don't exist
  • Automatically handles batching for large datasets

For complete examples of upserting embeddings in different vector stores, see the Upsert Embeddings guide.

Adding Metadata​

Vector stores support rich metadata (any JSON-serializable fields) for filtering and organization. Since metadata is stored with no fixed schema, use consistent field naming to avoid unexpected query results.

Important: Metadata is crucial for vector storage - without it, you'd only have numerical embeddings with no way to return the original text or filter results. Always store at least the source text as metadata.

// Store embeddings with rich metadata for better organization and filtering
await store.upsert({
  indexName: "myCollection",
  vectors: embeddings,
  metadata: chunks.map((chunk) => ({
    // Basic content
    text: chunk.text,
    id: chunk.id,

    // Document organization
    source: chunk.source,
    category: chunk.category,

    // Temporal metadata
    createdAt: new Date().toISOString(),
    version: "1.0",

    // Custom fields
    language: chunk.language,
    author: chunk.author,
    confidenceScore: chunk.score,
  })),
});

Key metadata considerations:

  • Be strict with field naming - inconsistencies like 'category' vs 'Category' will affect queries
  • Only include fields you plan to filter or sort by - extra fields add overhead
  • Add timestamps (e.g., 'createdAt', 'lastUpdated') to track content freshness

Best Practices​

  • Create indexes before bulk insertions
  • Use batch operations for large insertions (the upsert method handles batching automatically)
  • Only store metadata you'll query against
  • Match embedding dimensions to your model (e.g., 1536 for text-embedding-3-small)

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