Design database for cascading dropdown without circular reference

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C#

SQL

entity-framework

I have run into a circular reference when designing a database to store the data behind cascading drop downs. I am creating an object with properties that depend on each other the relationships seem like they would inevitable result in a circular reference.

For example I want to add a new piece of equipment to the database. The equipment has a type and sub-type. I pick the type then pick a sub-type that is filtered by that type. The new equipment then has a reference to the Type and Sub-Type which also have a relationship. So I end up with a triangular/circular reference (I would include an image but I cannot figure that out).

In this situation do people normally have the Equipment references only the SubType?
That would solve this instance since the Sub-Types only have one Type, but what if it was a many-to-many relationship between Type and Sub-Type?
What if the cascade has more than two choices that depend on each other? (maybe with cars Make, Model, trim, engine)

What I have tried:

Using C# and EF to create my database my (simplified) objects look like:

public class Equipment
{
       [Key]
       public Guid EquipmentId { get; set; }
       public string Description{ get; set; }

       /***** Relationship and Navigation *****/
       public Guid EquipmentTypeId { get; set; }
       public EquipmentType EquiptmentType { get; set; }
       public Guid EquipmentSubTypeId { get; set; }
       public EquipmentSubType EquipmentSubType { get; set; }
}

public class EquipmentType
{
        [Key]
        public Guid EquipmentTypeId { get; set; }
        public string Description{ get; set; }

        /***** Relationship and Navigation *****/
        public ICollection Equipment { get; set; }
        public ICollection EquipmentSubType { get; set; }
}

public class EquipmentSubType
{
        [Key]
        public Guid EquipmentSubTypeId { get; set; }
        public string Description{ get; set; }

        /***** Relationship and Navigation *****/
        public Guid EquipmentTypeId { get; set; }
        public EquipmentType EquipmentType { get; set; }
        public ICollection Equipment { get; set; }
}