I have several instances of state where I need to support actions that duplicate some slice of state. For example, my product is a survey builder, so when I duplicate a question, I'd also like to duplicate its answers, rather than have multiple questions pointing to the same answer instances.
The state is normalized:
questionsById: {
q01: {
...
answers: ["a01"],
...
}
}
answersById: {
a01: {...}
}
When dispatching an action of QUESTION_DUPLICATE, I'd like to also duplicate any answers. Currently my QUESTION_DUPLICATE action creator also creates a mapped list of new answer keys, and then the answer reducer consumes this.
This pattern seems unwieldy to me, especially when considering the possibility of deeper duplications (for example, duplicating a Page, which contains Questions, which contain Answers...). Is there a better pattern for deeply duplicating normalized data?
The answer may revolve around how you normally handle normalizing and denormalizing your data. For example, in my blog post Practical Redux, Part 8: Form Draft Data Management, I reuse my existing normalization logic (which leverages the redux-orm library) to copy an item to be edited between the "current" and "draft" slices in my state. So, similarly, one approach would be to denormalize the question you want to duplicate, and then re-normalize it (in either the action creator or the reducer, as you see fit).
I settled on using normalizr & I came up with a recursive duplicator function. It accepts an entity, schema, and keygen function, & recursively updates any nested entities based on the schemata by giving them new ids. In the base case (when there are no further nested entities) it will return the basic thing with its key updated.
const duplicator = (entity, schema, keygen) => {
const newEntity = {
...entity,
[schema._idAttribute]: keygen(entity, schema)
};
if (Object.keys(schema.schema).length === 0) {
return newEntity;
}
return Object.keys(schema.schema).reduce(
(acc, nestedKey) => {
if (!entity.hasOwnProperty(nestedKey)) {
return acc;
}
if (!Array.isArray(schema.schema[nestedKey])) {
return {
...acc,
[nestedKey]: duplicator(
entity[nestedKey],
schema.schema[nestedKey],
keygen
)
};
}
return {
...acc,
[nestedKey]: acc[nestedKey].map((nestedEntity, index) =>
duplicator(nestedEntity, schema.schema[nestedKey][0], keygen)
)
};
},
{ ...newEntity }
);
};
export default duplicator;
This currently doesn't support the schema.Array setup of normalizr for multiple entity types in an array. I'm not currently using schema.Array and this case would be pretty non-trivial to support, but I'll consider it in the future.
I'd like to know how to handle specific use case with redux reducer. To give an example, say I have a form with a DataGrid/Table. On Edit button click from a row, I want to open the popup with seleted row-data. After editing the data, On Popup-Submit button click, I want to update the Table/DataGrid (i.e. DataGrid will now should have the edited values).
I've written two separate Components
1. MainPage.js and its corresponding reducer MainPageReducer (Employee List)
2. PopupPage.js and its corresponding reducer PopupPageReducer (Selected Employee)
How these two reducers share the state?
You may need to read this first
http://redux.js.org/docs/basics/UsageWithReact.html
The main concept is that through the connect function, you would simply map needed properties of your state to the properties of your component i.e MapStateToProps. So in your case, imagine that your state, for contrived purposes, is structed like so:
{employees: {employees: {1: {id: 1, name: 'Foo'}}, editedEmployeeId: 1}
You could map the array of employees to an employees property for your EmployeeList component whilst also mapping a dispatch function, named editEmployee(id) to a click function on each row in the table.
You could map [the employee with the associated editedEmployeeId] to the individual employee in your employees array for your popup component
It may be efficient to just use one reducer instead of two.
Specifically, if you're making an update to an individual employee then you would call an EDIT_EMPLOYEE action and then a SAVE_EMPLOYEE action on save. After the SAVE_EMPLOYEE action, then, I assume, you'd call a post method, and then react-redux would re-render your entire list.
It could look like this:
function employees(state = {editedEmployeeId: undefined, employees = []}, action) {
switch(action.type) {
case EDIT_EMPLOYEE:
return Object.assign({}, state, {editedEmployee: action.employee_id})
case SAVE_EMPLOYEE:
return Object.assign({}, state, {employees: action.employees});
default:
return state;
}
}
There are great holes in my answer because the question you're asking might be too broad; I'm presuming you don't fully understand how the connect, subscribe, and dispatch functions work.
Like one of the comments said, reducers don't share state. They simply take the previous version of your state and return another version of it.
Anyways, hope this helps. Read the redux docs!
I have 2 reducers that I use and combine them. In the first reducer, I have something that gets all the initial data (which is also relevant for the second reducer).
How do I use the data in the state that I initialize/set from the first reducer to the second one?
function reducer1(state = initialState, action = '') {
switch (action.type) {
case constants.INITIAL_DATA:
returnstate.set('data', fromJS(document.data));
....
Then I combine both of those reducers and I want to access "data" from both of them (or pass the data as initialState to the second reducer).
A reducer should return a plain object and have no side effects. If you want the same state available in 2 different action.type cases, then they either need to be in the same function, or a parent function needs to pass the state to different reducer functions, like so:
function parentReducer(state = {}, action = '') {
switch (action.type) {
case CASE_ONE:
return childReducer1(state, action)
case CASE_TWO:
return childReducer2(state, action)
default:
return state
}
}
But this brings up an important design point: each (top-level) reducer represents a distinct branch of the Redux state tree, and you can probably always design your data store in a way that different parts of the tree don't need to be shared. In Redux (check out the DevTools), you have a single object, and the top-level keys of this object are the names of your top-level reducer functions.
Basically, if you perceive a need to set a different state in a reducer, so other reducers can use that, it evidence of a need to rethink the store's design.
This is a question about state in Redux js. I have an array list in the state:
{
list: list
}
According to Redux document, I should not modify the state in the reducer. I want to append a new item to the list. Should I clone the list, or simply append the new item to the list:
let newList = state.list;
newList.push(newItem);
return {
list: newList
}
The above code actually modify the original state, because the newList is the same list as "state.list". Is this an OK practice, or should I use immutableJS to clone the list?
You are mutating the list here and giving back the same reference. I believe something like below should make a new copy
return { list: [...state.list, newItem] }
It's not mandatory to use immutableJS. but you need to make sure not to mutate the state.
take a look at deep-freeze library to make sure that your state is not being edited.
this video from Dan (creator of redux) should help as well
If you aren't keen on the spread operator, you can also do:
var newList = list.slice(); // copy the array
newList.concat(['item']);
return {
list : newList
}
I'm trying to test out Firebase to allow users to post comments using push. I want to display the data I retrieve with the following;
fbl.child('sell').limit(20).on("value", function(fbdata) {
// handle data display here
}
The problem is the data is returned in order of oldest to newest - I want it in reversed order. Can Firebase do this?
Since this answer was written, Firebase has added a feature that allows ordering by any child or by value. So there are now four ways to order data: by key, by value, by priority, or by the value of any named child. See this blog post that introduces the new ordering capabilities.
The basic approaches remain the same though:
1. Add a child property with the inverted timestamp and then order on that.
2. Read the children in ascending order and then invert them on the client.
Firebase supports retrieving child nodes of a collection in two ways:
by name
by priority
What you're getting now is by name, which happens to be chronological. That's no coincidence btw: when you push an item into a collection, the name is generated to ensure the children are ordered in this way. To quote the Firebase documentation for push:
The unique name generated by push() is prefixed with a client-generated timestamp so that the resulting list will be chronologically-sorted.
The Firebase guide on ordered data has this to say on the topic:
How Data is Ordered
By default, children at a Firebase node are sorted lexicographically by name. Using push() can generate child names that naturally sort chronologically, but many applications require their data to be sorted in other ways. Firebase lets developers specify the ordering of items in a list by specifying a custom priority for each item.
The simplest way to get the behavior you want is to also specify an always-decreasing priority when you add the item:
var ref = new Firebase('https://your.firebaseio.com/sell');
var item = ref.push();
item.setWithPriority(yourObject, 0 - Date.now());
Update
You'll also have to retrieve the children differently:
fbl.child('sell').startAt().limitToLast(20).on('child_added', function(fbdata) {
console.log(fbdata.exportVal());
})
In my test using on('child_added' ensures that the last few children added are returned in reverse chronological order. Using on('value' on the other hand, returns them in the order of their name.
Be sure to read the section "Reading ordered data", which explains the usage of the child_* events to retrieve (ordered) children.
A bin to demonstrate this: http://jsbin.com/nonawe/3/watch?js,console
Since firebase 2.0.x you can use limitLast() to achieve that:
fbl.child('sell').orderByValue().limitLast(20).on("value", function(fbdataSnapshot) {
// fbdataSnapshot is returned in the ascending order
// you will still need to order these 20 items in
// in a descending order
}
Here's a link to the announcement: More querying capabilities in Firebase
To augment Frank's answer, it's also possible to grab the most recent records--even if you haven't bothered to order them using priorities--by simply using endAt().limit(x) like this demo:
var fb = new Firebase(URL);
// listen for all changes and update
fb.endAt().limit(100).on('value', update);
// print the output of our array
function update(snap) {
var list = [];
snap.forEach(function(ss) {
var data = ss.val();
data['.priority'] = ss.getPriority();
data['.name'] = ss.name();
list.unshift(data);
});
// print/process the results...
}
Note that this is quite performant even up to perhaps a thousand records (assuming the payloads are small). For more robust usages, Frank's answer is authoritative and much more scalable.
This brute force can also be optimized to work with bigger data or more records by doing things like monitoring child_added/child_removed/child_moved events in lieu of value, and using a debounce to apply DOM updates in bulk instead of individually.
DOM updates, naturally, are a stinker regardless of the approach, once you get into the hundreds of elements, so the debounce approach (or a React.js solution, which is essentially an uber debounce) is a great tool to have.
There is really no way but seems we have the recyclerview we can have this
query=mCommentsReference.orderByChild("date_added");
query.keepSynced(true);
// Initialize Views
mRecyclerView = (RecyclerView) view.findViewById(R.id.recyclerView);
mManager = new LinearLayoutManager(getContext());
// mManager.setReverseLayout(false);
mManager.setReverseLayout(true);
mManager.setStackFromEnd(true);
mRecyclerView.setHasFixedSize(true);
mRecyclerView.setLayoutManager(mManager);
I have a date variable (long) and wanted to keep the newest items on top of the list. So what I did was:
Add a new long field 'dateInverse'
Add a new method called 'getDateInverse', which just returns: Long.MAX_VALUE - date;
Create my query with: .orderByChild("dateInverse")
Presto! :p
You are searching limitTolast(Int x) .This will give you the last "x" higher elements of your database (they are in ascending order) but they are the "x" higher elements
if you got in your database {10,300,150,240,2,24,220}
this method:
myFirebaseRef.orderByChild("highScore").limitToLast(4)
will retrive you : {150,220,240,300}
In Android there is a way to actually reverse the data in an Arraylist of objects through the Adapter. In my case I could not use the LayoutManager to reverse the results in descending order since I was using a horizontal Recyclerview to display the data. Setting the following parameters to the recyclerview messed up my UI experience:
llManager.setReverseLayout(true);
llManager.setStackFromEnd(true);
The only working way I found around this was through the BindViewHolder method of the RecyclerView adapter:
#Override
public void onBindViewHolder(final RecyclerView.ViewHolder holder, int position) {
final SuperPost superPost = superList.get(getItemCount() - position - 1);
}
Hope this answer will help all the devs out there who are struggling with this issue in Firebase.
Firebase: How to display a thread of items in reverse order with a limit for each request and an indicator for a "load more" button.
This will get the last 10 items of the list
FBRef.child("childName")
.limitToLast(loadMoreLimit) // loadMoreLimit = 10 for example
This will get the last 10 items. Grab the id of the last record in the list and save for the load more functionality. Next, convert the collection of objects into and an array and do a list.reverse().
LOAD MORE Functionality: The next call will do two things, it will get the next sequence of list items based on the reference id from the first request and give you an indicator if you need to display the "load more" button.
this.FBRef
.child("childName")
.endAt(null, lastThreadId) // Get this from the previous step
.limitToLast(loadMoreLimit+2)
You will need to strip the first and last item of this object collection. The first item is the reference to get this list. The last item is an indicator for the show more button.
I have a bunch of other logic that will keep everything clean. You will need to add this code only for the load more functionality.
list = snapObjectAsArray; // The list is an array from snapObject
lastItemId = key; // get the first key of the list
if (list.length < loadMoreLimit+1) {
lastItemId = false;
}
if (list.length > loadMoreLimit+1) {
list.pop();
}
if (list.length > loadMoreLimit) {
list.shift();
}
// Return the list.reverse() and lastItemId
// If lastItemId is an ID, it will be used for the next reference and a flag to show the "load more" button.
}
I'm using ReactFire for easy Firebase integration.
Basically, it helps me storing the datas into the component state, as an array. Then, all I have to use is the reverse() function (read more)
Here is how I achieve this :
import React, { Component, PropTypes } from 'react';
import ReactMixin from 'react-mixin';
import ReactFireMixin from 'reactfire';
import Firebase from '../../../utils/firebaseUtils'; // Firebase.initializeApp(config);
#ReactMixin.decorate(ReactFireMixin)
export default class Add extends Component {
constructor(args) {
super(args);
this.state = {
articles: []
};
}
componentWillMount() {
let ref = Firebase.database().ref('articles').orderByChild('insertDate').limitToLast(10);
this.bindAsArray(ref, 'articles'); // bind retrieved data to this.state.articles
}
render() {
return (
<div>
{
this.state.articles.reverse().map(function(article) {
return <div>{article.title}</div>
})
}
</div>
);
}
}
There is a better way. You should order by negative server timestamp. How to get negative server timestamp even offline? There is an hidden field which helps. Related snippet from documentation:
var offsetRef = new Firebase("https://<YOUR-FIREBASE-APP>.firebaseio.com/.info/serverTimeOffset");
offsetRef.on("value", function(snap) {
var offset = snap.val();
var estimatedServerTimeMs = new Date().getTime() + offset;
});
To add to Dave Vávra's answer, I use a negative timestamp as my sort_key like so
Setting
const timestamp = new Date().getTime();
const data = {
name: 'John Doe',
city: 'New York',
sort_key: timestamp * -1 // Gets the negative value of the timestamp
}
Getting
const ref = firebase.database().ref('business-images').child(id);
const query = ref.orderByChild('sort_key');
return $firebaseArray(query); // AngularFire function
This fetches all objects from newest to oldest. You can also $indexOn the sortKey to make it run even faster
I had this problem too, I found a very simple solution to this that doesn't involved manipulating the data in anyway. If you are rending the result to the DOM, in a list of some sort. You can use flexbox and setup a class to reverse the elements in their container.
.reverse {
display: flex;
flex-direction: column-reverse;
}
myarray.reverse(); or this.myitems = items.map(item => item).reverse();
I did this by prepend.
query.orderByChild('sell').limitToLast(4).on("value", function(snapshot){
snapshot.forEach(function (childSnapshot) {
// PREPEND
});
});
Someone has pointed out that there are 2 ways to do this:
Manipulate the data client-side
Make a query that will order the data
The easiest way that I have found to do this is to use option 1, but through a LinkedList. I just append each of the objects to the front of the stack. It is flexible enough to still allow the list to be used in a ListView or RecyclerView. This way even though they come in order oldest to newest, you can still view, or retrieve, newest to oldest.
You can add a column named orderColumn where you save time as
Long refrenceTime = "large future time";
Long currentTime = "currentTime";
Long order = refrenceTime - currentTime;
now save Long order in column named orderColumn and when you retrieve data
as orderBy(orderColumn) you will get what you need.
just use reverse() on the array , suppose if you are storing the values to an array items[] then do a this.items.reverse()
ref.subscribe(snapshots => {
this.loading.dismiss();
this.items = [];
snapshots.forEach(snapshot => {
this.items.push(snapshot);
});
**this.items.reverse();**
},
For me it was limitToLast that worked. I also found out that limitLast is NOT a function:)
const query = messagesRef.orderBy('createdAt', 'asc').limitToLast(25);
The above is what worked for me.
PRINT in reverse order
Let's think outside the box... If your information will be printed directly into user's screen (without any content that needs to be modified in a consecutive order, like a sum or something), simply print from bottom to top.
So, instead of inserting each new block of content to the end of the print space (A += B), add that block to the beginning (A = B+A).
If you'll include the elements as a consecutive ordered list, the DOM can put the numbers for you if you insert each element as a List Item (<li>) inside an Ordered Lists (<ol>).
This way you save space from your database, avoiding unnecesary reversed data.