How can I filter nested items of IQueryable<T>?

Question

I am building an expression of type Expression<Func<Project, bool>> which returns the correct IQueryable<Project> from the database. IQueryable<Project> has a nested collection of SubProjects that I would like to filter as well. It looks something like this

Can this be done with one call to the database?

For example:

Expression<Func<Project, bool>> projectFilter = FilterEnabled();

projectFilter = projectFilter.And(GetProjectsByOrganization());

var projectData = GetProjectsAsQueryable(projectFilter); //returns correct projects

This is what I would like to do:

Expression<Func<Project, bool>> projectFilter = FilterEnabled();

projectFilter = projectFilter.And(GetProjectsByOrganization())
                             .And(GetSubProjectsByStartDate());

var projectData = GetProjectsAsQueryable(projectFilter); //returns correct projects and the filtered sub projects by start date

The GetProjectsByOrganization is as follows

public Expression<Func<Project, bool>> GetProjectByOrganization()
{
    var organizationIDs = new List<Guid>();

    if (FilterCriteria.OrganiazaitonId != null)
        organizationIDs = OrganizationRepository.GetParentAndChildrenOrganizationIds(FilterCriteria.OrganiazaitonId.Value).ToList();

    //...

    return prj => FilterCriteria.OrganiazaitonId == null || organizationIDs.Contains(prj.OrganizationID.Value);
}

How can I add an Expression<Func<SubProject, bool>> to the filter? If not what alternatives do I have?

Answer 1

You should be able to use Expression.AndAlso to combine all 3 Expressions to a new one. With Expression.PropertyOrField you can pass your SubProject instead your Project as Parameter:

    static Expression<Func<Project, bool>> CombineFilterExpression(
        Expression<Func<Project, bool>> firstProjectFilter, 
        Expression<Func<Project, bool>> secondProjectFilter,
        Expression<Func<SubProject, bool>> subProjectFilter
    )
    {
        //Create Project Parameter
        var param = Expression.Parameter(typeof(Project));
        //Create && Expression
        var body = Expression.AndAlso(
            Expression.Invoke(firstProjectFilter, param),
            Expression.AndAlso( //Create second && Expression
                Expression.Invoke(secondProjectFilter, param),
                //Pass SubProject instead of Project
                Expression.Invoke(subProjectFilter, Expression.PropertyOrField(param, nameof(Project.SubProject)))
            )
        );
        //Make Lambda with Project parameter
        return Expression.Lambda<Func<Project, bool>>(body, param);
    }