I am currently making an application that requires linking a Soap Formula to it’s Oils for later calculations.

The Join Class is called FormulaOil and has both ids

    public int FormulaId { get; set; }
    public Formula? Formula { get; set; }

    public int OilId { get; set; }
    public Oil? Oil { get; set; }

    public double Percentage { get; set; }
    public double Weight { get; set; }

and on Formula I have these Navigation Properties

    public virtual ICollection<FormulaOil> FormulaOils { get; set; } = new List<FormulaOil>();
    public virtual IEnumerable<Oil> Oils { get; set; } = new List<Oil>();

for Oil similarly

    public virtual ICollection<FormulaOil> FormulaOils { get; set; } = new List<FormulaOil>();
    public virtual ICollection<Formula> Formulas { get; set; } = new List<Formula>();

This to me seems to be matching the instruction given on the many to many section of the docs of entity framework https://learn.microsoft.com/en-us/ef/core/modeling/relationships/many-to-many

Yet when unit tested I seem to not be able to get the results correctly and the Oils navigation remains empty. Unit testing is done with an inMemoryDatabase.

        private readonly AppDbContext ctx;

        public UnitTest1()
        {
            var options = new DbContextOptionsBuilder<AppDbContext>().UseInMemoryDatabase(Guid.NewGuid().ToString()).Options;
            ctx = new AppDbContext(options, TestAuthenticatedUser.Instance);
        }


        [Fact]
        public void EnsureOilListReflectsWhatIsInOilIngrients()
        {
            var oil = new Oil()
            {
                Name = "Oil 1",
            };

            ctx.Oils.Add(oil);

            var formula = new Formula() { 
                Name = "Formula 1"
            };

            ctx.Formulas.Add(formula);

            ctx.SaveChanges();

            var oilIngrefient = new FormulaOil()
            {
                OilId = oil.Id,
                FormulaId = formula.Id,
            };

            ctx.OilIngredients.Add(oilIngrefient);
            ctx.SaveChanges();

            formula = ctx.Formulas.Include(x => x.FormulaOils).First();


            Assert.Equal(formula!.FormulaOils.Count, formula.Oils.Count());
        }

Am I missing some details in this?