An error was generated for warning 'Microsoft.EntityFrameworkCore.Query.InvalidIncludePathError': Unable to find navigation specified in string based include path

Question: What does this error mean? "InvalidOperationException: An error was generated for warning 'Microsoft.EntityFrameworkCore.Query.InvalidIncludePathError': Unable to find navigation 'CartItems' specified in string based include path 'CartItems'. This exception can be suppressed or logged by passing event ID 'CoreEventId.InvalidIncludePathError' to the 'ConfigureWarnings' method in 'DbContext.OnConfiguring' or 'AddDbContext'."

[Note]: When a Developer sees an InvalidOperationException, this simply means that the cause of an error was due to invalid arguments. (source: learn.microsoft.com, google.com accessed 11/5/2022)

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Answer: There is a possibility that you have used an "Include" directive wrong. See the notes below to make sure that you have provided the Navigation in the Data Model. The term "Navigation" in context means a Navigation Property in the Data Modal, take a look at the code below:

public class SomeMainModel {

//This a Property in SomeMainModel Class
public string id {get; set;}

//This is a Navigation Property referencing MySubModel Class
public List<MySubModel> mySubModel {get; set} = new List(MySubModel);
}

//This another Class separate from SomeMainModel
public class partial MySubModel {
//Properties come here
}

The code shown above contains a Navigation Property that allows you to use the "Include" directive in your LINQ Queries to include a list of MySubModel when retrieving Entities for MyMainModel:

var myWholeModel = _context.MyMainModel.Include(a=> a.MySubModel).ToList();

[Tip]: Make sure that you step through by placing Break-Points in your code to verify that the result is as expected.