Entity Framework core .Include() issue

I'm not sure if you've seen the accepted answer to this question, but the problem is to do with how the JSON Serializer deals with circular references. Full details and links to more references can be found at the above link, and I'd suggest digging into those, but in short, adding the following to startup.cs will configure the serializer to ignore circular references:

services.AddMvc()
    .AddJsonOptions(options => {
        options.SerializerSettings.ReferenceLoopHandling = ReferenceLoopHandling.Ignore;
    });

I test your code, this problem exist in my test. in this post LINK Proposed that use data projection. for your problem Something like the following, is work.

[HttpGet]
public dynamic Get()
{
    var dbContext = new ApplicationContext();

    var result = dbContext.Companies
        .Select(e => new { e.CompanyName, e.Id, e.Employees, e.Admins })
        .ToList();

    return result;
}

Lazy loading is not yet possible with EF Core. Refer here.

Alternatively you can use eager loading.

Read this article

Below is the extension method i have created to achieve the eager loading.

Extension Method:

public static IQueryable<TEntity> IncludeMultiple<TEntity, TProperty>(
    this IQueryable<TEntity> source,
    List<Expression<Func<TEntity, TProperty>>> navigationPropertyPath) where TEntity : class
{
    foreach (var navExpression in navigationPropertyPath)
    {
        source= source.Include(navExpression);
    }
    return source.AsQueryable();
}

Repository Call:

public async Task<TEntity> FindOne(ISpecification<TEntity> spec)
{
    return await Task.Run(() => Context.Set<TEntity>().AsQueryable().IncludeMultiple(spec.IncludeExpression()).Where(spec.IsSatisfiedBy).FirstOrDefault());
}

Usage:

List<object> nestedObjects = new List<object> {new Rules()};

ISpecification<Blog> blogSpec = new BlogSpec(blogId, nestedObjects); 

var challenge = await this._blogRepository.FindOne(blogSpec);

Dependencies:

public class BlogSpec : SpecificationBase<Blog>
{
    readonly int _blogId;
    private readonly List<object> _nestedObjects;

    public ChallengeSpec(int blogid, List<object> nestedObjects)
    {
        this._blogId = blogid;
        _nestedObjects = nestedObjects;
    }

    public override Expression<Func<Challenge, bool>> SpecExpression
    {
        get { return blogSpec => blogSpec.Id == this._blogId; }
    }

    public override List<Expression<Func<Blog, object>>> IncludeExpression()
    {
        List<Expression<Func<Blog, object>>> tobeIncluded = new List<Expression<Func<Blog, object>>>();
        if (_nestedObjects != null)
            foreach (var nestedObject in _nestedObjects)
            {
                if (nestedObject is Rules)
                {
                    Expression<Func<Blog, object>> expr = blog => blog.Rules;
                    tobeIncluded.Add(expr);
                }
                
            }

        return tobeIncluded;
    }
}

Will be glad if it helps. Please note this is not a production ready code.

Make sure you are using Include from "Microsoft.EntityFrameworkCore" And Not from "System.Data.Entity"