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Middleware.md

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Middleware

RxDB supports middleware-hooks like mongoose. Middleware (also called pre and post hooks) are functions which are passed control during execution of asynchronous functions. The hooks are specified on RxCollection-level and help to create a clear what-happens-when-structure of your code.

Hooks can be defined to run parallel or as series one after another. Hooks can be synchronous or asynchronous when they return a Promise. To stop the operation at a specific hook, throw an error.

List

RxDB supports the following hooks:

  • preInsert
  • postInsert
  • preSave
  • postSave
  • preRemove
  • postRemove

Why is there no validate-hook?

Different to mongoose, the validation on document-data is running on the field-level for every change to a document. This means if you set the value lastName of a RxDocument, then the validation will only run on the changed field, not the whole document. Therefore it is not usefull to have validate-hooks when a document is written to the database.

Use Cases

Middleware are useful for atomizing model logic and avoiding nested blocks of async code. Here are some other ideas:

  • complex validation
  • removing dependent documents
  • asynchronous defaults
  • asynchronous tasks that a certain action triggers
  • triggering custom events
  • notifications

Usage

Insert

An insert-hook recieves the data-object of the new document.

lifecycle

  • RxCollection.insert is called
  • preInsert series-hooks
  • preInsert parallel-hooks
  • schema validation runs
  • new document is written to database
  • postInsert series-hooks
  • postInsert parallel-hooks
  • event is emmited to RxDatabase and RxCollection

preInsert

// series
myCollection.preInsert(function(documentData){

}, false);

// parallel
myCollection.preInsert(function(documentData){

}, true);

// async
myCollection.preInsert(function(documentData){
  return new Promise(res => setTimeout(res, 100));
}, false);

// stop the insert-operation
myCollection.preInsert(function(documentData){
  throw new Error('stop');
}, false);

postInsert

// series
myCollection.postInsert(function(documentData){

}, false);

// parallel
myCollection.postInsert(function(documentData){

}, true);

// async
myCollection.postInsert(function(documentData){
  return new Promise(res => setTimeout(res, 100));
}, false);

Save

A save-hook receives the document which is saved.

lifecycle

  • RxDocument.save is called
  • preSave series-hooks
  • preSave parallel-hooks
  • updated document is written to database
  • postSave series-hooks
  • postSave parallel-hooks
  • event is emmited to RxDatabase and RxCollection

preSave

// series
myCollection.preSave(function(doc){
  doc.anyField = 'anyValue';
}, false);

// parallel
myCollection.preSave(function(doc){

}, true);

// async
myCollection.preSave(function(doc){
  return new Promise(res => setTimeout(res, 100));
}, false);

// stop the save-operation
myCollection.preSave(function(doc){
  throw new Error('stop');
}, false);

postSave

// series
myCollection.postSave(function(doc){

}, false);

// parallel
myCollection.postSave(function(doc){

}, true);

// async
myCollection.postSave(function(doc){
  return new Promise(res => setTimeout(res, 100));
}, false);

Remove

An remove-hook recieves the document which is removed.

lifecycle

  • RxDocument.remove is called
  • preRemove series-hooks
  • preRemove parallel-hooks
  • deleted document is written to database
  • postRemove series-hooks
  • postRemove parallel-hooks
  • event is emmited to RxDatabase and RxCollection

preSave

// series
myCollection.preRemove(function(doc){

}, false);

// parallel
myCollection.preRemove(function(doc){

}, true);

// async
myCollection.preRemove(function(doc){
  return new Promise(res => setTimeout(res, 100));
}, false);

// stop the remove-operation
myCollection.preRemove(function(doc){
  throw new Error('stop');
}, false);

postRemove

// series
myCollection.postRemove(function(doc){

}, false);

// parallel
myCollection.postRemove(function(doc){

}, true);

// async
myCollection.postRemove(function(doc){
  return new Promise(res => setTimeout(res, 100));
}, false);

If you are new to RxDB, you should continue here