97 lines
2.7 KiB
Markdown
97 lines
2.7 KiB
Markdown
# ManyToMany
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`ManyToMany(to, through)` has required parameters `to` and `through` that takes target and relation `Model` classes.
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Sqlalchemy column and Type are automatically taken from target `Model`.
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* Sqlalchemy column: class of a target `Model` primary key column
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* Type (used for pydantic): type of a target `Model`
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## Defining Models
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```Python hl_lines="32 49-50"
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--8<-- "../docs_src/relations/docs002.py"
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```
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Create sample data:
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```Python
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guido = await Author.objects.create(first_name="Guido", last_name="Van Rossum")
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post = await Post.objects.create(title="Hello, M2M", author=guido)
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news = await Category.objects.create(name="News")
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```
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### add
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```python
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# Add a category to a post.
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await post.categories.add(news)
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# or from the other end:
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await news.posts.add(post)
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```
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!!!warning
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In all not None cases the primary key value for related model **has to exist in database**.
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Otherwise an IntegrityError will be raised by your database driver library.
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### remove
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Removal of the related model one by one.
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Removes also the relation in the database.
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```python
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await news.posts.remove(post)
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```
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### clear
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Removal of all related models in one call.
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Removes also the relation in the database.
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```python
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await news.posts.clear()
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```
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### QuerysetProxy
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Reverse relation exposes QuerysetProxy API that allows you to query related model like you would issue a normal Query.
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To read which methods of QuerySet are available read below [querysetproxy][querysetproxy]
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## related_name
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By default, the related_name is generated in the same way as for the `ForeignKey` relation (class.name.lower()+'s'),
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but in the same way you can overwrite this name by providing `related_name` parameter like below:
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```Python
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categories: Optional[Union[Category, List[Category]]] = ormar.ManyToMany(
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Category, through=PostCategory, related_name="new_categories"
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)
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```
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!!!warning
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When you provide multiple relations to the same model `ormar` can no longer auto generate
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the `related_name` for you. Therefore, in that situation you **have to** provide `related_name`
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for all but one (one can be default and generated) or all related fields.
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[queries]: ./queries.md
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[querysetproxy]: ./queryset-proxy.md
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[get]: ./queries.md#get
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[all]: ./queries.md#all
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[create]: ./queries.md#create
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[get_or_create]: ./queries.md#get_or_create
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[update_or_create]: ./queries.md#update_or_create
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[filter]: ./queries.md#filter
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[exclude]: ./queries.md#exclude
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[select_related]: ./queries.md#select_related
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[prefetch_related]: ./queries.md#prefetch_related
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[limit]: ./queries.md#limit
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[offset]: ./queries.md#offset
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[count]: ./queries.md#count
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[exists]: ./queries.md#exists
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[fields]: ./queries.md#fields
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[exclude_fields]: ./queries.md#exclude_fields
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[order_by]: ./queries.md#order_by |