Add mongoose migration guide

docs/pages/upgrade-v3/mongoose.md

@@ -3,3 +3,171 @@ title: "Upgrade your Mongoose project to v3"
 ---
 
 # Upgrade your Mongoose project to v3
+
+Read this guide carefully as some parts depend on your current structure (**especially the collection names**), and feel free to ask questions on our Discord server if you have any questions.
+
+## Update the adapter
+
+The Mongoose adapter has been replaced with the MongoDB adapter.
+
+```
+npm install @lucia-auth/adapter-mongodb@beta
+```
+
+Initialize the adapter:
+
+```ts
+import { MongoDBAdapter } from "@lucia-auth/adapter-mongodb";
+import mongoose from "mongoose";
+
+const adapter = new MongodbAdapter(
+	mongoose.connection.collection("sessions"),
+	mongoose.connection.collection("users")
+);
+```
+
+## Update the session collection
+
+Replace the `idle_expires` field with `expires_at` and update the Mongoose schema accordingly.
+
+```ts
+db.sessions.updateMany({}, [
+	{
+		$set: {
+			expires_at: { $toDate: "$idle_expires" }
+		}
+	},
+	{
+		$unset: ["idle_expires", "active_expires"]
+	}
+]);
+```
+
+```ts
+import mongoose from "mongoose";
+
+const Session = mongoose.model(
+	"Session",
+	new mongoose.Schema(
+		{
+			_id: {
+				type: String,
+				required: true
+			},
+			user_id: {
+				type: String,
+				required: true
+			},
+			expires_at: {
+				type: Date,
+				required: true
+			}
+		} as const,
+		{ _id: false }
+	)
+);
+```
+
+## Replace the key collection
+
+Keys have been removed. You can keep using them but you may want to update your schema to better align with MongoDB.
+
+### OAuth accounts
+
+This database command adds a `github_id` field to users with a GitHub account based on the key collection.
+
+```ts
+db.users.aggregate([
+	{
+		$lookup: {
+			from: "keys",
+			localField: "_id",
+			foreignField: "user_id",
+			as: "github_accounts",
+			pipeline: [
+				{
+					$match: {
+						_id: {
+							$regex: /^github:/
+						}
+					}
+				}
+			]
+		}
+	},
+	{
+		$match: {
+			$expr: {
+				$gt: [{ $size: "$github_accounts" }, 0]
+			}
+		}
+	},
+	{
+		$set: {
+			github_id: {
+				$replaceOne: {
+					input: { $arrayElemAt: ["$github_accounts._id", 0] },
+					find: "github:",
+					replacement: ""
+				}
+			}
+		}
+	},
+	{
+		$unset: ["github_accounts"]
+	},
+	{
+		$merge: {
+			into: "users",
+			whenMatched: "merge"
+		}
+	}
+]);
+```
+
+### Password accounts
+
+This database command moves the `hashed_password` field from the keys collection to the users collection.
+
+```ts
+db.users.aggregate([
+	{
+		$lookup: {
+			from: "keys",
+			localField: "_id",
+			foreignField: "user_id",
+			as: "password_accounts",
+			pipeline: [
+				{
+					$match: {
+						hashed_password: {
+							$ne: null
+						}
+					}
+				}
+			]
+		}
+	},
+	{
+		$match: {
+			$expr: {
+				$gt: [{ $size: "$password_accounts" }, 0]
+			}
+		}
+	},
+	{
+		$set: {
+			hashed_password: { $arrayElemAt: ["$password_accounts.hashed_password", 0] }
+		}
+	},
+	{
+		$unset: ["password_accounts"]
+	},
+	{
+		$merge: {
+			into: "users",
+			whenMatched: "merge"
+		}
+	}
+]);
+```