feat: modify mongo index function (#19)

index.go

@@ -0,0 +1,72 @@
+package xmongo
+
+import (
+	"context"
+	"fmt"
+
+	"go.mongodb.org/mongo-driver/bson"
+	"go.mongodb.org/mongo-driver/mongo"
+	"go.mongodb.org/mongo-driver/mongo/options"
+)
+
+// doc: https://www.mongodb.com/docs/v7.0/tutorial/manage-indexes/#minimize-performance-impact-with-a-temporary-index
+func ModifyIndex(ctx context.Context, collection *mongo.Collection, name string, newIndex mongo.IndexModel) error {
+	// 0. find the existing index
+	indexes, err := collection.Indexes().ListSpecifications(ctx, options.ListIndexes())
+	if err != nil {
+		return fmt.Errorf("list indexes: %w", err)
+	}
+
+	var existingIndex *mongo.IndexSpecification
+	for _, i := range indexes {
+		if i.Name == name {
+			existingIndex = i
+		}
+	}
+
+	if existingIndex == nil {
+		return fmt.Errorf("index %s not found", name)
+	}
+
+	rawkeys, err := existingIndex.KeysDocument.Elements()
+	if err != nil {
+		return fmt.Errorf("keys document: %w", err)
+	}
+
+	var keys []bson.E
+	for _, key := range rawkeys {
+		keys = append(keys, bson.E{Key: key.Key(), Value: 1})
+	}
+
+	const dummyField = "_tmp_dummy_field"
+	keys = append(keys, bson.E{Key: dummyField, Value: 1})
+
+	tmpIndexName := fmt.Sprintf("temp_index_%s", name)
+
+	// 1. create a temporary index
+	if _, err := collection.Indexes().CreateOne(ctx, mongo.IndexModel{
+		Keys: keys,
+		Options: &options.IndexOptions{
+			Name: &tmpIndexName,
+		},
+	}); err != nil {
+		return fmt.Errorf("create temp index: %w", err)
+	}
+
+	// 2. drop the existing index
+	if _, err := collection.Indexes().DropOne(ctx, name); err != nil {
+		return fmt.Errorf("drop index %s: %w", name, err)
+	}
+
+	// 3. create the new index
+	if _, err := collection.Indexes().CreateOne(ctx, newIndex); err != nil {
+		return fmt.Errorf("create new index: %w", err)
+	}
+
+	// 4. drop the temporary index
+	if _, err := collection.Indexes().DropOne(ctx, tmpIndexName); err != nil {
+		return fmt.Errorf("drop temp index: %w", err)
+	}
+
+	return nil
+}