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add a tip about @ManyToMany

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Gavin King 2023-06-29 03:04:47 +02:00
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documentation/src/main/asciidoc/introduction/Entities.adoc
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@ -1221,6 +1221,22 @@ If the association is bidirectional, we annotate the unowned side `@OneToOne(map
A unidirectional many-to-many association is represented as a collection-valued attribute.
It always maps to a separate _association table_ in the database.
[TIP]
====
It tends to happen that a many-to-many association eventually turns out to be an entity in disguise.
Suppose we start with a nice clean many-to-many.
Later on, it's quite likely that we'll discover some additional information which comes attached to the association, and so the association table needs some extra columns.
For example, imagine that we needed to report the percentage contribution of each author to a book.
That information naturally belongs to the association table.
We can't easily store it as an attribute of of `Book`, nor as an attribute of `Author`.
When this happens, we need to change our Java model, usually introducing a new entity class which maps the association table.
In our example, it we might call this entity something like `BookAuthorship`, and it would have `@OneToMany` associations to both `Author` and `Book`.
We can evade this disruption by simply avoiding the use of `@ManyToMany`, and representing every logical many-to-many association using an intermediate entity right from the start.
====
A many-to-many association must be annotated `@ManyToMany`:
[source,java]
@ -1314,6 +1330,7 @@ So we may expand our taxonomy with:
There's actually two new kinds of mapping here: `@Array` mappings, and `@ElementCollection` mappings.
[%unbreakable]
[CAUTION]
// .These sorts of mappings are overused
====
@ -1338,6 +1355,7 @@ We might represent this in our `Event` entity as an attribute of type `DayOfWeek
Since the number of elements of this array or list is upper bounded by 7, this is a reasonable case for the use of an `ARRAY`-typed column.
It's hard to see much value in storing this collection in a separate table.
[%unbreakable]
.Learning to not hate SQL arrays
****
For a long time, we thought arrays were a kind of weird and warty thing to add to the relational model, but recently we've come to realize that this view was overly closed-minded.
@ -1410,6 +1428,7 @@ create table Event_daysOfWeek (
Which is fine, but it's still a mapping we prefer to avoid.
[%unbreakable]
[WARNING]
// .This is not what we would do
====