The YouTube Subscription Model

Think about how YouTube subscriptions work:

• A channel on YouTube is just a user account
• When user A "subscribes" to user B's channel, there is a relationship between A (subscriber) and B (channel)
• Unsubscribing removes that relationship

The key insight is that there is no separate Channel entity. Channels are just users. So the Subscription document simply records: "this user subscribed to this other user."

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Subscription Schema Design

const subscriptionSchema = new Schema(
  {
    subscriber: {
      type: Schema.Types.ObjectId,
      ref: 'User',
      required: true,
    },
    channel: {
      type: Schema.Types.ObjectId,
      ref: 'User',
      required: true,
    },
  },
  { timestamps: true }
);

// Compound unique index: a user can only subscribe to a channel once
subscriptionSchema.index({ subscriber: 1, channel: 1 }, { unique: true });

Both subscriber and channel reference the same User collection — they are both user IDs. The field names clarify the role.

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Counting Subscribers of a Channel

// How many users have subscribed to a channel?
const subscriberCount = await Subscription.countDocuments({ channel: channelId });

If user B has 1000 subscribers, that means there are 1000 Subscription documents where channel === userB._id.

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Counting Channels a User Subscribes To

// How many channels has this user subscribed to?
const subscriptionCount = await Subscription.countDocuments({ subscriber: userId });

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Toggle Subscribe / Unsubscribe

const toggleSubscription = asyncHandler(async (req, res) => {
  const { channelId } = req.params;
  const subscriberId = req.user._id;

  if (channelId.toString() === subscriberId.toString()) {
    throw new ApiError(400, 'You cannot subscribe to your own channel');
  }

  const existingSubscription = await Subscription.findOne({
    subscriber: subscriberId,
    channel: channelId,
  });

  let message;
  if (existingSubscription) {
    // Already subscribed — unsubscribe
    await Subscription.findByIdAndDelete(existingSubscription._id);
    message = 'Unsubscribed successfully';
  } else {
    // Not subscribed — subscribe
    await Subscription.create({ subscriber: subscriberId, channel: channelId });
    message = 'Subscribed successfully';
  }

  return res.status(200).json(new ApiResponse(200, {}, message));
});

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Aggregation: Get Both Counts + isSubscribed in One Query

Instead of three separate database queries, use aggregation to get everything at once:

const channelStats = await User.aggregate([
  { $match: { _id: new mongoose.Types.ObjectId(channelId) } },

  // Join: get all subscribers of this channel
  {
    $lookup: {
      from: 'subscriptions',
      localField: '_id',
      foreignField: 'channel',
      as: 'subscribers',
    },
  },

  // Join: get all channels this user subscribes to
  {
    $lookup: {
      from: 'subscriptions',
      localField: '_id',
      foreignField: 'subscriber',
      as: 'subscribedTo',
    },
  },

  {
    $addFields: {
      subscribersCount: { $size: '$subscribers' },
      subscribedToCount: { $size: '$subscribedTo' },
      // Check if the requesting user is in the subscribers list
      isSubscribed: {
        $in: [new mongoose.Types.ObjectId(req.user._id), '$subscribers.subscriber'],
      },
    },
  },

  {
    $project: {
      subscribersCount: 1,
      subscribedToCount: 1,
      isSubscribed: 1,
    },
  },
]);

This pattern powers the channel profile page: subscriber count, who the channel subscribes to, and a subscribe/unsubscribe button — all from a single aggregation.

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Why This Design Avoids a Separate Channel Collection

Some developers create a separate Channel collection, but this leads to:

• Data duplication (user name/avatar stored in both User and Channel)
• Complex sync logic (update both User and Channel when profile changes)
• More complex queries (join User → Channel → Subscription)

With the "subscriptions are user-to-user" design:

• One source of truth (User collection)
• Subscriptions are lightweight relationship documents
• Easy to query: filter by subscriber or channel field

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Index Strategy for Performance