A Helper for Implementing Relay's Cursor-Based Pagination with Sequelize

sequelize_relay_cursor_pagination.ts

const { fromGlobalId, toGlobalId } = require("graphql-relay");
import {FindOptions} from "sequelize";

export interface ICursorPageable {
    // Backward Paging Arguments
    before?: string;
    last?: number;
    
    // Forward Paging Arguments
    after?: string;
    first?: number;
}

/**
 * This method will mutate the incoming criteria object with paging arguments that align to the 
 * Relay's Paging interface (last/before, first/after). The query will be paged against the query's results, 
 * not physical identifiers.
 */
export function addCursorPagingCriteria(criteria: FindOptions, args: ICursorPageable) {
    const {first, last, after, before} = args;
    const isForwardPaging = !!first;
    const isBackwardPaging = !!last;
    const getGlobalId = cursor => parseInt(fromGlobalId(cursor).id, 10);
    
    if (isForwardPaging && isBackwardPaging) {
        throw new Error("cursor-based pagination cannot be forwards AND backwards");
    }
    if (isForwardPaging && before || isBackwardPaging && after) {
        throw new Error("paging must use either first/after or last/before");
    }
    if (isForwardPaging && first < 0 || isBackwardPaging && last < 0) {
        throw new Error("paging limit must be positive");
    }
    
    if (isForwardPaging) {
        criteria.limit = first;    
        if (after) {
            criteria.offset = getGlobalId(after) + 1 || 0;
        }
    } else if (isBackwardPaging) {
        criteria.limit = last;
        if (before) {
            let offset = getGlobalId(before) - last;
            // Check if this query underflows.  
            if (offset < 0) {
                // Adjust the limit with the underflow value
                criteria.limit = Math.max(last + offset, 0);
                offset = 0; 
            }
            criteria.offset = offset;
        }
    }
}

usage:

resolve(root, args) {
    const criteria = makeCriteria(args);
    addCursorPagingCriteria(criteria, args);                               
    return connectionFromPromisedArraySlice(
        repository.findAll(criteria),  // the repository adds a default order property to the criteria if none exists
        args, 
        {sliceStart: criteria.offset, arrayLength: Number.MAX_VALUE}
    );
}