CodeLiturgy.Dashboard/BlueWest.Api/Context/Templates/AddToListInEntity.csx

public static (bool, string, {returnTypeFullName}) Add{listEntityTypeName}To{entityTypeName}(  this {contextFullName} dbContext, 
            {keyTypeFullName} {keyVarName}, {listEntityCreateFullName} {listItemCreateVarName},   Expression<Func<{entityTypeFullName},bool>>[] duplicationValidations)
{
    var entityQuery = from aEntity in dbContext.{propertyName}
        where aEntity.{keyVarName} == {keyVarName}
        let itemsInList = aEntity.{entityListMemberName}
        select aEntity;
    
    var entity = entityQuery.FirstOrDefault();
    
    if (entity == null) return (false, $"{nameof(country)} Not found.", null);
    
    foreach (var duplicationValidation in duplicationValidations)
    {
        var entityToGet = dbContext.{entityListMemberName}.FirstOrDefault(duplicationValidation);
        if (entityToGet != null)
        {
            return (false, $"Duplication Validation failed: {nameof(duplicationValidation.Body.ToString)}", null);
        }
    }

    var newListItem = new {listEntityFullName}({listItemCreateVarName});
    entity.{entityListMemberName}.Add({listItemCreateVarName});
    var success = dbContext.SaveChanges() >= 0;
    return !success ? (false, "Error saving changes.", null) : (true, string.Empty, new {returnTypeFullName}(newListItem));
}