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+# Firestore Data Model Reference
+
+Firestore is a NoSQL, document-oriented database. Unlike a SQL database, there are no tables or rows. Instead, you store data in **documents**, which are organized into **collections**.
+
+## Document Data Model
+
+Data in Firestore is organized into documents, collections, and subcollections.
+
+### Documents
+A **document** is a lightweight record that contains fields, which map to values. Each document is identified by a name. A document can contain complex nested objects in addition to basic data types like strings, numbers, and booleans. Documents are limited to a maximum size of 1 MiB.
+
+Example document (e.g., in a `users` collection):
+```json
+{
+  "first": "Ada",
+  "last": "Lovelace",
+  "born": 1815
+}
+```
+
+### Collections
+Documents live in **collections**, which are containers for your documents. For example, you could have a `users` collection to contain your various users, each represented by a document.
+* Collections can only contain documents. They cannot directly contain raw fields with values, and they cannot contain other collections.
+* Documents within a collection can contain different fields.
+* You don't need to "create" or "delete" collections explicitly. After you create the first document in a collection, the collection exists. If you delete all of the documents in a collection, the collection no longer exists.
+
+### Subcollections
+Documents can contain subcollections natively. A subcollection is a collection associated with a specific document.
+For example, a user document in the `users` collection could have a `messages` subcollection containing message documents exclusively for that user. This creates a powerful hierarchical data structure.
+
+Data path example: `users/user1/messages/message1`
+
+## Collection Group Support
+
+A **collection group** consists of all collections with the same ID. By default, queries retrieve results from a single collection in your database. Use a collection group query to retrieve documents from a collection group instead of from a single collection.
+
+### Use Cases
+Collection group queries are useful when you want to query across multiple subcollections that share the same organizational structure.
+
+For example, imagine an app with a `landmarks` collection where each landmark has a `reviews` subcollection. If you want to find all 5-star reviews across *all* landmarks, it would involve checking many separate `reviews` subcollections. With a collection group, you can perform a single query against the `reviews` collection group.
+
+### Examples
+
+**Standard Query** (Single Collection):
+Find all 5-star reviews for a specific landmark.
+```javascript
+db.collection('landmarks/golden_gate_bridge/reviews').where('rating', '==', 5)
+```
+
+**Collection Group Query**:
+Find all 5-star reviews across *all* landmarks.
+```javascript
+db.collectionGroup('reviews').where('rating', '==', 5)
+```