Play with db constraints
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Weinstock
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# Bookshop Getting Started Sample
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# Foreign key constraints on DB
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This stand-alone sample introduces the essential tasks in the development of CAP-based services as also covered in the [Getting Started guide in capire](https://cap.cloud.sap/docs/get-started/in-a-nutshell).
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As model we use a simplified version of the bookshop scenario, we work with nodejs runtime.
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## Hypothetical Use Cases
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1. Build a service that allows to browse _Books_ and _Authors_.
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2. Books have assigned _Genres_, which are organized hierarchically.
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3. All users may browse books without login.
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4. All entries are maintained by Administrators.
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5. End users may order books (the actual order mgmt being out of scope).
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## Preparation
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## Running the Sample
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```sh
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npm run watch
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As we want to see the effects of the database constaints, we first need to switch off the integrity checks done by the node runtime. In the `package.json`:
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```json
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"cds": {
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...,
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"features": {
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"assert_integrity": false
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}
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}
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```
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## Content & Best Practices
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Then switch on constraint generation. In the `package.json`:
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```json
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"cds": {
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...,
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"cdsc": {
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"beta": {
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"foreignKeyConstraints": true
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}
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}
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}
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```
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| Links to capire | Sample files / folders |
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| --------------------------------------------------------------------------------------------------------- | ------------------------------------ |
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| [Project Setup & Layouts](https://cap.cloud.sap/docs/get-started/projects#sharing-and-reusing-content) | [`./`](./) |
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| [Domain Modeling with CDS](https://cap.cloud.sap/docs/guides/domain-models) | [`./db/schema.cds`](./db/schema.cds) |
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| [Defining Services](https://cap.cloud.sap/docs/guides/services#defining-services) | [`./srv/*.cds`](./srv) |
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| [Single-purposed Services](https://cap.cloud.sap/docs/guides/services#single-purposed-services) | [`./srv/*.cds`](./srv) |
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| [Providing & Consuming Providers](https://cap.cloud.sap/docs/guides/providing-services) | http://localhost:4004 |
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| [Using Databases](https://cap.cloud.sap/docs/guides/databases) | [`./db/data/*.csv`](./db/data) |
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| [Adding Custom Logic](https://cap.cloud.sap/docs/guides/service-impl) | [`./srv/*.js`](./srv) |
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| Adding Tests | [`./test`](./test) |
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| [Sharing for Reuse](https://cap.cloud.sap/docs/guides/reuse-and-compose) | [`./index.cds`](./index.cds) |
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## SQLite
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In SQLite, FK constraints can only be specified during the creation of a table. They cannot be altered for an existing table. So here we at least don't have issues with existing data.
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For SQLite, constraint checks need to be enabled by application at runtime for each db connection with the command
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```sql
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PRAGMA foreign_keys = ON;
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```
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We do this via a custom handler in `admin-service.js`.
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Now run the requests in `requests.http` (use `http://localhost:4004` as server URL). They behave as expected.
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But the error messages are not providing any details. They say "constraint violation", but they don't say which table/constraint is violated:
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```
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HTTP/1.1 500 Internal Server Error
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...
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{
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"error": {
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"code": "SQLITE_CONSTRAINT",
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"message": "SQLITE_CONSTRAINT: FOREIGN KEY constraint failed in: \nCOMMIT"
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}
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}
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```
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The runtime constraint checks do provide the name of the restricting association (but not the name of the entity where the association lives).
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## HANA
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Preparation:
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* logon to cf account (e.g. trial)
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* create HANA service instance: `cf create-service hanatrial hdi-shared bookshop-db`
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First make the deplyoment without constraints (switch off constraint generation in `package.json`).
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* Run `cds build --production`. No constraints are generated.
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* Deploy db model: `cf push -f gen/db`
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* Deploy nodejs app: `cf push -f gen/srv --random-route`. Take note of the app URL.
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Run the requests in `request.http` (use the URL from the previous step as server URL, prepend with `https://`).
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Make sure to introduce some data that violates the constraints. No errors are reported - as expected.
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Now switch on constraint generation in `package.json`.
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* Run `cds build --production`. The constraints are placed in separate `hdbconstraint` files in `gen\db\src\gen`. They are generated with `VALIDATED ON`.
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* Re-deploy db model: `cf push -f gen/db`. It fails, as the existing data violates the constraints.
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* Check logs: `cf logs bookshop-db-deployer --recent`.
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```
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Error: com.sap.hana.di.constraint: Could not create the database object [8250005]
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at "src/gen/sap.capire.bookshop.Books_author.hdbconstraint" (1:1)
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Error: com.sap.hana.di.constraint: Database error 7: : feature not supported: referential constraint violated by existing value [8201003]
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at "src/gen/sap.capire.bookshop.Books_author.hdbconstraint" (1:1)
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```
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You can see which constraint is violated, but there is no info about the offensive entries.
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Now use the requests from `request.http` to fix the entries that have been broken above.
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* Re-deploy db model: `cf push -f gen/db`. It should succeed.
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Again run the requests from `request.http`. This time all changes that would lead to a
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constraint violation fail.
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Inserting a book with non-existing author ID or changing a book's author ID to an invalid value yields
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the message
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```
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HTTP/1.1 500 Internal Server Error
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...
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{
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"error": {
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"code": "500",
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"message": "Internal Server Error"
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}
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}
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```
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When trying to delete a genre that is still referenced in a book we get
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```
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HTTP/1.1 462 status code 462
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...
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{
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"error": {
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"code": "462",
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"message": "failed on update or delete by foreign key constraint violation: TrexColumnUpdate failed on table '21F87859727E470E94917B690908A09D:SAP_CAPIRE_BOOKSHOP_GENRES' with error: DELETE on 21F87859727E470E94917B690908A09D:SAP_CAPIRE_BOOKSHOP_GENRES(ID) failed, because 1 rows with corresponding foreign keys still exist in 21F87859727E470E94917B690908A09D:SAP_CAPIRE_BOOKSHOP_BOOKS(GENRE_ID), rc=1536"
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}
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}
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```
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Note: when using `cdsc` to generate the constraints
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```
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cdsc Q --beta foreignKeyConstraints -d hana -s hdi -o dbstuff db\schema.cds srv\admin-service.cds
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```
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there will be constraints for e.g. `sap_common_Currencies_texts`. This cannot be deplyoed, as the corresponding table is removed from `cds` via the tree-shaking mechanism. These constraints need to be removed manually before deployment.
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## Todo
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* Provide a way to control parameters `VALIDATED` and `ENFORCED` of the constraints (only interesting for HANA, as in SQLite changing constraints after table generation is not possible).
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* Provide some tool to identify existing entries that violate constraints. We can generate a list of `SELECT` statements that find these entries, but how do we run it and show the results to the user?
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* Can runtimes improve the error messages?
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