module.exports = (event, model) => {
    const _ = require('lodash');
    const sequence = require('../../lib/promise/sequence');
    const api = require('../../api');

    const apiVersion = model.get('api_version') || 'v2';
    const docName = model.tableName;

    const ops = [];

    if (Object.keys(model.attributes).length) {
        let frame = {options: {previous: false, context: {user: true}}};

        ops.push(() => {
            return api.shared
                .serializers
                .handle
                .output(model, {docName: docName, method: 'read'}, api[apiVersion].serializers.output, frame)
                .then(() => {
                    return frame.response[docName][0];
                });
        });
    } else {
        ops.push(() => {
            return Promise.resolve({});
        });
    }

    if (Object.keys(model._previousAttributes).length) {
        ops.push(() => {
            const frame = {options: {previous: true, context: {user: true}}};

            return api.shared
                .serializers
                .handle
                .output(model, {docName: docName, method: 'read'}, api[apiVersion].serializers.output, frame)
                .then(() => {
                    return frame.response[docName][0];
                });
        });
    } else {
        ops.push(() => {
            return Promise.resolve({});
        });
    }

    sequence(ops)
        .then((results) => {
            const current = results[0];
            const previous = results[1];

            const changed = model._changed ? Object.keys(model._changed) : {};

            const payload = {
                [docName.replace(/s$/, '')]: {
                    current: current,
                    previous: _.pick(previous, changed)
                }
            };

            // @TODO: remove in v3
            // @NOTE: Our webhook format has changed, we still have to support the old format for subscribers events
            if ('subscriber.added' === event) {
                payload[docName] = [current];
            }

            if ('subscriber.deleted' === event) {
                payload[docName] = [previous];
            }

            return payload;
        });
};