如何从模型管理器访问实例？

Question

有人告诉我这没有意义，因为管理器对所有行（而不是单个实例）进行操作，但是我在django-taggit库中看到了要实现的目标。

此处：https://github.com/alex/django-taggit/blob/master/taggit/managers.py

安装过程如下：

<div>
    <label for="item">Item</label>
    <select id="item">
        <option value="">--- Select an Item ---</option>
        <option value="Sofa">Sofa</option>
        <option value="Chair">Chair</option>
    </select>
</div>

<div>
    <label for="type">Type</label>
    <select id="type"></select>
</div>

<div>
    <label for="color">Color</label>
    <select id="color"></select>
</div>

<script>

    var types = [];
    var colors = [];

    // Get a reference to the select boxs
    var item = document.getElementById('item');
    var type = document.getElementById('type');
    var color = document.getElementById('color');

    // Add an event handler for when an item is selected
    item.addEventListener('change', function () {

        i = item.options[item.selectedIndex].value;

        if (i == "Sofa") {
            types = [
                { text: "Full Size", value: "full-size" },
                { text: "Love Seat", value: "love-seat" }
            ];
        }

        if (i == "Chair") {
            types = [
                { text: "Standard", value: "standard" },
                { text: "Recliner", value: "recliner" }
            ];
        }

        setOptions(type, types);

    });

    // Add an event handler for when an type is selected
    type.addEventListener('change', function () {

        i = type.options[type.selectedIndex].value;
        console.log(type.options[type.selectedIndex].value);

        if (i == "full-size") {
            colors = [
                { text: "Brown", value: "brown" },
                { text: "White", value: "white" },
                { text: "Black", value: "black" }
            ];
        }

        if (i == "love-seat") {
            colors = [
                { text: "Tan", value: "tan" },
                { text: "Red", value: "red" },
                { text: "Copper", value: "copper" }
            ];
        }

        if (i == "standard") {
            colors = [
                { text: "Green", value: "green" },
                { text: "Yellow", value: "yellow" },
                { text: "Red", value: "red" }
            ];
        }

        if (i == "recliner") {
            colors = [
                { text: "Orange", value: "orange" },
                { text: "Purple", value: "purple" },
                { text: "Magenta", value: "magenta" }
            ];
        }

        setOptions(color, colors);

    });

    // Function to add Options
    function setOptions(select, opts) {

        // Clear existing values
        select.innerHTML = "";

        // Add a select option
        var elm = document.createElement('option');
        elm.appendChild(document.createTextNode("--- Select an Option"));
        select.appendChild(elm);

        for (o in opts) {
            var opt = opts[o];

            // create new option element
            var elm = document.createElement('option');

            // create text node to add to option element (opt)
            elm.appendChild(document.createTextNode(opt.text));

            // set value property of opt
            elm.value = opt.value;

            // add opt to end of select box (sel)
            select.appendChild(elm);
        }
    }

</script>

然后标记任何内容，只需执行以下操作即可

：

from django.db import models

from taggit.managers import TaggableManager

class Food(models.Model):
    # ... fields here

    tags = TaggableManager()

注意：apple.tags.add("red", "green", "fruit") 不是apple。

但是，当我自己尝试做时，我得到：Apple！

我的代码：

AttributeError: Manager isn't accessible via MyModelName instances

然后，我尝试按以下方式调用add方法：

from django.db import models


class TagManager(models.Manager):
    def add(self, *tags):
        print("Testing...")


class List(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    title = models.CharField(max_length=500)

    tags = TagManager()

我如何使其工作？