How to dynamically add items from different entities to lists in ASP.NET Core MVC

This is my first experience with lists in general and I have kinda complex scenario. I want to add an object of type Story that has a list of Sentences that can be added dynamically. Sentence has a one-to-one relationship with Image and another one-to-one relationship with Audio (that are optional to add). I managed to add the sentences list to the database along with the story object. But I have no idea where to start with the other two entities.

Here are the models for each entity:

public class Story
{
    public int StoryId { get; set; }
    public string Title { get; set; }
    public string Type { get; set; }
    public DateTime Date { get; set; }

    public virtual IEnumerable<Sentence> Sentences { get; set; } // one to many Story-Sentence
}

Sentence class:

public class Sentence
{
    public int Id { get; set; }
    public string SentenceText { get; set; }

    public virtual Audio Audio { get; set; } // one to one Sentence-Audio
    public virtual Image Image { get; set; } // one to one Sentence-Image
}

Image class:

public class Image
{
    [Key]
    [ForeignKey("Sentence")]
    public int Id { get; set; }
    public string ImageSelected { get; set; }
    public virtual Sentence Sentence { get; set; }
}

And the Audio class is exactly like the Image class. The view.

@model Story
<div id="editorRows">
    @foreach (var item in Model.Sentences)
    {
        <partial name="_SentenceEditor" model="item" />
    }
</div>
<a id="addItem" asp-action="BlankSentence" asp-controller="StoryTest">Add Sentence...</a> <br />
<input type="submit" value="Finished" />

The partial view

@model Sentence
<div class="editorRow">
    @using (Html.BeginCollectionItem("sentences"))
    {
        <span>Name: </span> @Html.EditorFor(m => m.SentenceText);
    }
    @using (Html.BeginCollectionItem("sentences"))
    {
        <span>Image: </span> @Html.EditorFor(m => m.Image.ImageSelected);
    }
    <a href="#" class="deleteRow">delete</a>
</div>

and I have some javascript that add and remove rows dynamically.

Finally in the Controller I'm just saving the model in the database

        [HttpPost]
    public async Task<IActionResult> AddTwo(Story model/*IEnumerable<Sentence> sentence*/)
    {
        if (ModelState.IsValid)
        {
            _db.Story.Add(model);
            await _db.SaveChangesAsync();
            return RedirectToAction("Index");
        }
        return View(model);
    }

In short, I want to add an Image and Audio along with the sentence. And also be able to access the entire row correctly for editing.

I made a demo based on your codes. You can store data in formData then use ajax pass it to the controller action.

Main View:

@model Story
<form asp-action="AddSentence" method="post">
    <div id="editorRows">
        <input type="hidden" name="" value="@Model.StoryId" />
        @foreach (var item in Model.Sentences)
        {
            <partial name="_SentenceEditor" model="item" />
        }
    </div>
    <a id="addItem" asp-action="BlankSentence" asp-controller="StoryTest">Add Sentence...</a>
    <br />
    <input type="submit" id="submit" value="Finished" />
</form>

@section scripts {
    <script>
        $("#submit").click(function (e) {
            e.preventDefault();
            var formData = new FormData();

            $("input[name='Audio.AudioSelected']").each(function (i) {
                var AudioSelected = $(this).val();
                formData.append("Sentences[" + i + "].Audio.AudioSelected", AudioSelected);

            });
            $("input[name='Image.ImageSelected']").each(function (i) {
                var ImageSelected = $(this).val();
                formData.append("Sentences[" + i + "].Image.ImageSelected", ImageSelected);

            });
            $("input[name='SentenceText']").each(function (i) {
                var SentenceText = $(this).val();
                formData.append("Sentences[" + i + "].SentenceText", SentenceText);

            });

            $.ajax({
                method: 'post',
                url: "StoryTest/AddSentence",
                data: formData,
                processData: false,
                contentType: false,
                success: function () {

                }
            });

        });

        $("#addItem").click(function () {
            $.ajax({
                url: this.href,
                cache: false,
                success: function (html) { $("#editorRows").append(html); }
            });
            return false;
        });

        $("a.deleteRow").on("click", function () {
            $(this).parents("div.editorRow:first").remove();
            return false;
        });
    </script>
}

Partial View:

@model Sentence
<div class="editorRow">

    <span>Name: </span> @Html.EditorFor(m => m.SentenceText)

    <span>Audio: </span> @Html.EditorFor(m => m.Audio.AudioSelected)

    <span>Image: </span> @Html.EditorFor(m => m.Image.ImageSelected)

    <a href="#" class="deleteRow">delete</a>
</div>

Controller:

public IActionResult Index()
{
    Story story = new Story
    {
        Sentences = new List<Sentence>
        {
            new Sentence { Id = 1, SentenceText = "AAA"
            , Audio = new Audio{ AudioSelected = "True"}
            , Image = new Image{ ImageSelected = "True"} },

            new Sentence { Id = 2, SentenceText = "BBB"
            , Audio = new Audio{ AudioSelected = "False"}
            , Image = new Image{ ImageSelected = "False"}},

            new Sentence { Id = 3, SentenceText = "CCC"
            , Audio = new Audio{ AudioSelected = "True"}
            , Image = new Image{ ImageSelected = "False"}}
        }
    };
    return View(story);
}

[HttpPost]
public async Task<IActionResult> AddSentence(Story model)
{
    if (ModelState.IsValid)
    {
        _db.Story.Add(model);
        await _db.SaveChangesAsync();
        return RedirectToAction("Index");
    }
    return View(model);
}

public IActionResult BlankSentence()
{
    return PartialView("_SentenceEditor", new Sentence());
}

Result: