Django Models for Hydroponic Garden Monitoring

Your model structure is on the right track, but I can suggest some improvements based on Django best practices. Here's a refined version of your models:

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from django.db import models

class PlantSpecies(models.Model):
    name = models.CharField(max_length=100, unique=True)
    low_pH = models.FloatField(help_text="Minimum recommended pH")
    high_pH = models.FloatField(help_text="Maximum recommended pH")
    
    def __str__(self):
        return self.name
    
    class Meta:
        verbose_name_plural = "Plant species"

class PlantInstance(models.Model):
    instance_id = models.CharField(max_length=20, unique=True)
    species = models.ForeignKey(PlantSpecies, on_delete=models.CASCADE, related_name='instances')
    started = models.DateField()
    ended = models.DateField(null=True, blank=True)
    
    def __str__(self):
        return f"{self.instance_id} ({self.species.name})"

class ActivityLog(models.Model):
    plant = models.ForeignKey(PlantInstance, on_delete=models.CASCADE, related_name='activities')
    timestamp = models.DateTimeField(auto_now_add=True)
    initial_pH = models.FloatField()
    final_pH = models.FloatField()
    notes = models.TextField(blank=True)
    
    def __str__(self):
        return f"Log for {self.plant.instance_id} on {self.timestamp.strftime('%Y-%m-%d %H:%M')}"
    
    class Meta:
        ordering = ['-timestamp']

Key Improvements:

1. Primary Keys: Django automatically creates an id field as the primary key, so you don't need to define log_id, etc. unless you have a specific reason to use custom IDs.

2. Field Names: Changed species_id to name and datetime to timestamp for clarity and to avoid confusion with actual IDs.

3. Related Names: Added related_name to ForeignKey fields, which makes querying easier (e.g., plant_instance.activities.all()).

4. String Representation: Added __str__ methods to make objects display nicely in the admin interface.

5. Meta Options: Added ordering to ActivityLog to show newest entries first and proper plural name for PlantSpecies.

6. Unique Constraints: Added unique=True to fields that should be unique.

7. Help Text: Added help_text for better documentation in the admin interface.

This structure will work well with Django's built-in admin interface and make querying straightforward. For example, to get all activities for a specific plant:

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plant = PlantInstance.objects.get(instance_id='003')
activities = plant.activities.all()  # Gets all activities for this plant

Or to get the pH range for a plant:

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plant = PlantInstance.objects.get(instance_id='003')
ph_range = (plant.species.low_pH, plant.species.high_pH)