Django: ManyToManyField with intermediary model error

Question

Django is not managing our databases for us, therefor I created the table RulesetRuleMap to handle the ManyToMany relationship between Ruleset and Rule: Each Ruleset can consist of multiple Rules and each Rule can be used in multiple Rulesets.

Models

class Rule(models.Model):
    id = models.BigAutoField(primary_key=True)
    percentage_of_total = models.FloatField(blank=False, null=False)
    _rule_parameter = models.ForeignKey('RuleParameter', models.DO_NOTHING, blank=False, null=False)

    class Meta:
        managed = False
        db_table = '_rule'


class Ruleset(models.Model):
    id = models.BigAutoField(primary_key=True)
    name = models.CharField(max_length=300, blank=False, null=False)
    description = models.CharField(max_length=300, blank=False, null=False)
    rules = models.ManyToManyField('Rule', through="RulesetRuleMap")

    class Meta:
        managed = False
        db_table = '_ruleset'


class RulesetRuleMap(models.Model):
    id = models.BigAutoField(primary_key=True)
    _rule = models.ForeignKey('Rule', models.CASCADE)
    _ruleset = models.ForeignKey('Ruleset', models.CASCADE)

    class Meta:
        managed = False
        db_table = '_ruleset_rule_map'

Serializers

class RulesetRuleMapSerializer(serializers.ModelSerializer):
    class Meta:
        model = db_models.RulesetRuleMap
        fields = '__all__'


class RuleSerializer(serializers.ModelSerializer):
    class Meta:
        model = db_models.Rule
        fields = '__all__'


class RulesetSerializer(serializers.ModelSerializer):
    rules = RuleSerializer(many=True)
    class Meta:
        model = db_models.Ruleset
        fields = '__all__'

    def create(self, validated_data):
        rules_data = validated_data.pop('rules')
        ruleset = db_models.Ruleset.objects.create(**validated_data)
        rules_storage =[]
        for rule_data in rules_data:
            rule, created = db_models.Rule.objects.get_or_create(**rule_data)
            rules_storage.append(rule)
        ruleset.rules.add(*rules_storage, through_defaults={})
        return ruleset

On a homepage the user can add/modify a Ruleset and add/modify the assosiated Rules. On submission we receive a payload like this:

{
  "id": None,
  "name": "Split_50.0_Param1_50.0_Param2",
  "description": "test",
  "rules": [
    {
      "id": None,
      "percentage_of_total": "50",
      "tc_rule_parameter": "3"
    },
    {
      "id": None,
      "percentage_of_total": "50",
      "tc_rule_parameter": "2"
    }
  ]
}

As described in Djange REST Framework I defined a custom create() for the nested RulesetSerializer to handle the creation of multiple objects. According to Django one should be able to

use add(), create(), or set() to create relationships, as long as you specify through_defaults for any required fields.

When executing ruleset.rules.add(*rules_storage, through_defaults={}) I get the error

{TypeError}add() got an unexpected keyword argument 'through_defaults'

When executing ruleset.rules.add(*rules_storage) I get the error

{AttributeError}Cannot use add() on a ManyToManyField which specifies an intermediary model.Use database_models.TcRulesetRuleMap's Manager instead.

Is there a mistake in my model and/or serializer set up or is there a bug in django?

Answer 1

I misread the django docs. I'm on django 2.1 and the through fields were only introduced in 2.2.