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Update docu on DB constraints #1383

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Update docu on DB constraints #1383

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a1c6a7e
Update docu on DB constraints
stewsk Oct 28, 2024
1d32460
Update section on DB constraints
stewsk Nov 14, 2024
1702e12
Merge branch 'main' into fk-constraints
stewsk Nov 14, 2024
62af73f
Merge branch 'main' into fk-constraints
stewsk Dec 12, 2024
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20 changes: 18 additions & 2 deletions guides/databases.md
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Expand Up @@ -819,7 +819,9 @@ Enable generation of foreign key constraints on the database with:
Referential constraints on H2 cannot be defined as "deferred", which is needed for database constraints within CAP.
:::

With that switched on, foreign key constraints are generated for managed to-one associations. For example, given this model:
With that switched on, foreign key constraints are generated for managed to-one associations, if both
the source and the target entity of the association are tables in the database.
For example, given this model:

```cds
entity Books {
Expand Down Expand Up @@ -903,11 +905,25 @@ CREATE TABLE Books_texts (
```

::: warning Database constraints aren't intended for checking user input
Instead, they protect the integrity of your data in the database layer against programming errors. If a constraint violation occurs, the error messages coming from the database aren't standardized by the runtimes but presented as-is.

As developer of an application, you have to ensure in your implementation that the data written to the database is consistent.
For example, user input must be checked by the application.
→ Use [`@assert.target`](providing-services#assert-target) for corresponding input validations.

Database constraints act as a last line of defense that protect the integrity of your data
in the database layer against programming errors, like an assertion.
If a constraint violation occurs, the error message coming from the database isn't standardized by the runtimes but presented as-is.
Such an error is a (late) signal to the _application developer_ that the implementation of the app itself has deficiencies.

:::

On SAP HANA, database constraints are internally implemented via triggers on both
related tables. HANA allows no more than 1024 triggers for a table,
resulting in limits for managed associations with database constraints switched on:
An entity can't contain more than 1024 such associations, and a given entity cannot be
the target of more than 1024 associations. If you run into this limitation, deactivate
database level constraints (globally or selectively for some associations)
and implement another mechanism to assert data integrity.


## Using Native Features { #native-db-functions}
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