LinkEntitiesByBatchId

using Stylelabs.M.Base.Querying;
using Stylelabs.M.Base.Querying.Linq;
using Stylelabs.M.Framework.Essentials.LoadOptions;
using Stylelabs.M.Sdk.Contracts.Base;
using Stylelabs.M.Sdk.Contracts.Querying;
using System.Globalization;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

MClient.Logger.Info("Getting Entities");

int totalMappedProducts = default;
int totalSkipped = default;
int total = default;

CultureInfo defaultCulture = await MClient.Cultures.GetDefaultCultureAsync();

var query = Query.CreateQuery(entities => from e in entities
                                            where e.DefinitionName == "definitionName"
                                            && e.Property("BatchId") == "batch1" // Remove this line if you want to run it on all entities of the definition
                                            select e);

var result = await MClient.Querying.QueryIdsAsync(query);
long totalNumberInCH = result.TotalNumberOfResults;
MClient.Logger.Info($"Total Number of  entities - {totalNumberInCH}");

IIdIterator iterator = MClient.Querying.CreateEntityIdIterator(query);
while (await iterator.MoveNextAsync())
{
    var entities = iterator.Current.Items;
    foreach (var item in entities)
    {
        ++total;
        var entity = await MClient.Entities.GetAsync(item);
        if(entity == null)
        {
            MClient.Logger.Info($"Entity {item} not found.");
            continue; 
        }

        MClient.Logger.Info($"Processing {entity.Identifier} - {entity.Id} . Total processed - {total}. Total in CH - {totalNumberInCH}.");

        #region Load Options

        var propertyLoadOption = new PropertyLoadOption("Catalog", "ProductName");
        var relationLoadOption = new RelationLoadOption("Products");

        var isLoaded = await entity.LoadMembersAsync(propertyLoadOption, relationLoadOption);
        if (!isLoaded)
            continue;

        #endregion

        var relation = entity.GetRelation("Products", RelationRole.Parent);
        var productName = entity.GetPropertyValue("ProductName");
        MClient.Logger.Info(productName.ToString());
        var productId = await GetProductId(productName.ToString().Trim());
        if (!productId.HasValue)
        {
            MClient.Logger.Info($"Product {productName} was not found in CH");
            continue;
        }

        relation.SetIds(new List<long> { productId.Value });
        var id = await MClient.Entities.SaveAsync(entity);
        MClient.Logger.Info($"Relation [{productId}] added to {entity.DefinitionName} | {entity.Id}");
        MClient.Logger.Info(productName.ToString());
    }
}

async Task<long?> GetProductId(string product)
{
    MClient.Logger.Info($"Getting Product - {product}");
    var query = Query.CreateQuery(entities =>
        from e in entities
        where e.Property("ProductName") == product
        && e.DefinitionName == "M.PCM.Product"
        select e);
    var productId = await MClient.Querying.SingleIdAsync(query);
    MClient.Logger.Info($"Product {product} found. Id - {productId}");

    return productId;
}