Django REST: Dynamically add Model Fields - django

Question

I'm working on a Django Rest project where I'm given two MySQL tables:
metrics: Contain a row for each potential metric
daily_data: Contains a row for each data entry where the column names refer to metrics from the 'metrics' table
What I want to do now, is creating new entries in 'metrics' which should be automatically added to existing 'daily_data' entries (with a default value) and displayed on the website.
Here is how the current models looks like:
class Metrics(model.Model):
metric_id = models.CharField(max_length=255, primary_key=True)
is_main_metric = models.BooleanField(default=False)
name = models.CharField(max_length=255, blank=False, null=False)
description = models.CharField(max_length=255, blank=False, null=False)
lower_bound = models.FloatField(default=0.0, null=False)
upper_bound = models.FloatField(default=0.0, null=False)
class Meta:
verbose_name_plural = "Metrics"
db_table = "metrics"
class DailyData(models.Model):
location = models.CharField(max_length=255, blank=False, null=False)
date = models.DateField(blank=False, null=False)
# then a static field for each metric is added that corresponds to a 'metric_id' in the table 'metrics':
metric_01 = models.FloatField(default=0.0, null=False)
metric_02 = models.FloatField(default=0.0, null=False)
metric_03 = models.FloatField(default=0.0, null=False)
...
class Meta:
verbose_name_plural = "Daily Data"
db_table = "daily_data"
Later on, the Javascript code iterates over all 'metrics' to display them with the corresponding values from a requested 'daily_data' entry. Here is a small example:
let resp = await axios.get(`${API_URL}/daily_data/?location=berlin&date=2021-01-07`);
let data = resp.data[0];
METRICS.forEach(metric => {
let name = metric.name;
let description = metric.description;
let value = data[metric.metric_id];
$content.append(
` <div class="row">
<span>${name}:</span>
<span>${value}</span>
<span>${description}"</span>
</div> `
);
...
}
For the case that all metrics are pre-defined, the application is running fine. If I want to add a new metric, I create a new row in the database table 'metrics', then add the field manually to the 'DailyData' model from above, and finally restart the server.
However, my problem now is that I need the possibility to add new metrics dynamically. I.e. if a user adds a new metric (for example with a POST request), the metric should be added as a column to all existing 'daily_data' entries and should be displayed as an additional field on the website.
The intention is basically something like this (I know that this won't work, but just to get the idea):
def onNewMetricCreation(newMetric):
metric_id = newMetric.metric_id
new_field = models.FloatField(default=0.0, null=False)
DailyData.appendField(metric_id, new_field)
Is there a way to achieve this and add these model fields dynamically? Or is my whole data structure faulty for this case?
Edit: To solve the problem I've actually changed my data structure a bit. I've added a MetricsData model that connects the DailyData with the Metrics and contains the corresponding values. This allows each DailyData object to have a different number of metrics and new ones can be added easily.
The new models look like this:
class DailyData(models.Model):
location = models.ForeignKey("Locations", on_delete=models.CASCADE, blank=False, null=False)
date = models.DateField(blank=False, null=False)
class MetricsData(models.Model):
data_entry = models.ForeignKey("DailyData", on_delete=models.CASCADE, related_name="data_entry")
metric = models.ForeignKey("Metrics", on_delete=models.CASCADE)
value = models.FloatField(default=0.0, null=False)
class Metrics(models.Model):
metric_id = models.CharField(max_length=255, primary_key=True)
...

Answer 1

If I understood you correct I belive you're looking for a ForeignKey(). You would add this to your model:
class DailyData(models.Model):
metrics = models.ForeignKey(Metrics, on_delete=models.CASCADE)
Go inside django admin and I think you'll understand how ForeignKeys work. It's a reference to the metrics instance. Ps. don't add this field dynamically, that's probably impossible. But with this you can simply add another row.
So if you reference an instance of metrics. And then change that. all daily_data that references that will be "changed" since they're still referenceing the same instance.
If you need to reference more the one metrics use ManyToMany

Answer 2

I strongly recommend that you add a Foreign Key for DailyData to Metrics model.
class Metrics(model.Model):
...
related_day = models.ForeignKey(DailyData, on_delete=models.CASCADE, related_name="metrics", related_query_name="metrics", null=True)
Now you also need to add a signal to trigger after creating a metric to connect that metric to its related data.
#receiver(post_save, sender=Metrics)
def add_to_daily_data(sender, instance, created, **kwargs):
if created:
# Put your logic to add a specific metric to a daily data
Also, this way you can access all metrics data related to specific DailyData objects hassle-free.
daily_data.metrics.all()