I have a table mapping departments and teams in MySQL.
I want to retrieve array of teams for each department.
For example, if I have two departments depA (teams teamAA, teamAB), depB (teams teamBA, teamBB, teamBC), I want to obtain following JSON
[
{
code: "depA",
teams: ["teamAA", "teamAB"]
},
{
code: "depB",
teams: ["teamBA", "teamBB", "teamBC"]
}
]
I am able to use GROUP_CONCAT to obtain a concatenated string but, I want a JSON array so that the sequelize.js library I am using can implicitly parse the entire data structure to js object.
It can be done like so,
SELECT `departmentCode` AS `code`, JSON_ARRAY(GROUP_CONCAT(DISTINCT `teamCode`)) AS `teams`
I need to make a query where I can get "Place" table, but I have to exclude from that query all "Place" that are already part of the "LocationGroupPlace" table.
I have a class called Place and other called LocationGroupPlace
each LocationGroupPlace can contain many Places
these are the relations
in Place.php file
public function locationGroupPlace()
{
return $this->hasMany(LocationGroupPlace::class, 'place_id');
}
in LocationGroupPlace.php file
public function place(): \Illuminate\Database\Eloquent\Relations\BelongsTo
{
return $this->belongsTo(Place::class);
}
I am trying to retrieve 5 items from "Place" that do not exist in "LocationGroupPlace", that is, the IDs of "Place" are not in the table of "LocationGroupPlace", for example these IDs
I'm trying to get the first 5 Place with all the table from LocationGroupPlace with a subquery (for test my subquery)
public function getUsedPlacesForGroups()
{
return Place::with(['locationGroupPlace' => function ($query){
$query->get();
}])->limit(5)->get();
}
but I can't get LocationGroupPlace, the relations section is empty
Note: locationGroupPlace is the function in Place
How can I get all LocationGroupPlace? my final idea is to exclude all "Places" included in "LocationGroupPlace"
If you want the places that doesn't exist in LocationGroupPlace then you can use doesntHave
$places= Place::doesntHave('locationGroupPlace')->limit(5)->get();
See Querying Relationship Absence
{
"movies": {
"movie1": {
"genre": "comedy",
"name": "As good as it gets",
"lead": "Jack Nicholson"
},
"movie2": {
"genre": "Horror",
"name": "The Shining",
"lead": "Jack Nicholson"
},
"movie3": {
"genre": "comedy",
"name": "The Mask",
"lead": "Jim Carrey"
}
}
}
I am a Firebase newbie. How can I retrieve a result from the data above where genre = 'comedy' AND lead = 'Jack Nicholson'?
What options do I have?
Using Firebase's Query API, you might be tempted to try this:
// !!! THIS WILL NOT WORK !!!
ref
.orderBy('genre')
.startAt('comedy').endAt('comedy')
.orderBy('lead') // !!! THIS LINE WILL RAISE AN ERROR !!!
.startAt('Jack Nicholson').endAt('Jack Nicholson')
.on('value', function(snapshot) {
console.log(snapshot.val());
});
But as #RobDiMarco from Firebase says in the comments:
multiple orderBy() calls will throw an error
So my code above will not work.
I know of three approaches that will work.
1. filter most on the server, do the rest on the client
What you can do is execute one orderBy().startAt()./endAt() on the server, pull down the remaining data and filter that in JavaScript code on your client.
ref
.orderBy('genre')
.equalTo('comedy')
.on('child_added', function(snapshot) {
var movie = snapshot.val();
if (movie.lead == 'Jack Nicholson') {
console.log(movie);
}
});
2. add a property that combines the values that you want to filter on
If that isn't good enough, you should consider modifying/expanding your data to allow your use-case. For example: you could stuff genre+lead into a single property that you just use for this filter.
"movie1": {
"genre": "comedy",
"name": "As good as it gets",
"lead": "Jack Nicholson",
"genre_lead": "comedy_Jack Nicholson"
}, //...
You're essentially building your own multi-column index that way and can query it with:
ref
.orderBy('genre_lead')
.equalTo('comedy_Jack Nicholson')
.on('child_added', function(snapshot) {
var movie = snapshot.val();
console.log(movie);
});
David East has written a library called QueryBase that helps with generating such properties.
You could even do relative/range queries, let's say that you want to allow querying movies by category and year. You'd use this data structure:
"movie1": {
"genre": "comedy",
"name": "As good as it gets",
"lead": "Jack Nicholson",
"genre_year": "comedy_1997"
}, //...
And then query for comedies of the 90s with:
ref
.orderBy('genre_year')
.startAt('comedy_1990')
.endAt('comedy_2000')
.on('child_added', function(snapshot) {
var movie = snapshot.val();
console.log(movie);
});
If you need to filter on more than just the year, make sure to add the other date parts in descending order, e.g. "comedy_1997-12-25". This way the lexicographical ordering that Firebase does on string values will be the same as the chronological ordering.
This combining of values in a property can work with more than two values, but you can only do a range filter on the last value in the composite property.
A very special variant of this is implemented by the GeoFire library for Firebase. This library combines the latitude and longitude of a location into a so-called Geohash, which can then be used to do realtime range queries on Firebase.
3. create a custom index programmatically
Yet another alternative is to do what we've all done before this new Query API was added: create an index in a different node:
"movies"
// the same structure you have today
"by_genre"
"comedy"
"by_lead"
"Jack Nicholson"
"movie1"
"Jim Carrey"
"movie3"
"Horror"
"by_lead"
"Jack Nicholson"
"movie2"
There are probably more approaches. For example, this answer highlights an alternative tree-shaped custom index: https://stackoverflow.com/a/34105063
If none of these options work for you, but you still want to store your data in Firebase, you can also consider using its Cloud Firestore database.
Cloud Firestore can handle multiple equality filters in a single query, but only one range filter. Under the hood it essentially uses the same query model, but it's like it auto-generates the composite properties for you. See Firestore's documentation on compound queries.
I've written a personal library that allows you to order by multiple values, with all the ordering done on the server.
Meet Querybase!
Querybase takes in a Firebase Database Reference and an array of fields you wish to index on. When you create new records it will automatically handle the generation of keys that allow for multiple querying. The caveat is that it only supports straight equivalence (no less than or greater than).
const databaseRef = firebase.database().ref().child('people');
const querybaseRef = querybase.ref(databaseRef, ['name', 'age', 'location']);
// Automatically handles composite keys
querybaseRef.push({
name: 'David',
age: 27,
location: 'SF'
});
// Find records by multiple fields
// returns a Firebase Database ref
const queriedDbRef = querybaseRef
.where({
name: 'David',
age: 27
});
// Listen for realtime updates
queriedDbRef.on('value', snap => console.log(snap));
var ref = new Firebase('https://your.firebaseio.com/');
Query query = ref.orderByChild('genre').equalTo('comedy');
query.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot movieSnapshot : dataSnapshot.getChildren()) {
Movie movie = dataSnapshot.getValue(Movie.class);
if (movie.getLead().equals('Jack Nicholson')) {
console.log(movieSnapshot.getKey());
}
}
}
#Override
public void onCancelled(FirebaseError firebaseError) {
}
});
Frank's answer is good but Firestore introduced array-contains recently that makes it easier to do AND queries.
You can create a filters field to add you filters. You can add as many values as you need. For example to filter by comedy and Jack Nicholson you can add the value comedy_Jack Nicholson but if you also you want to by comedy and 2014 you can add the value comedy_2014 without creating more fields.
{
"movies": {
"movie1": {
"genre": "comedy",
"name": "As good as it gets",
"lead": "Jack Nicholson",
"year": 2014,
"filters": [
"comedy_Jack Nicholson",
"comedy_2014"
]
}
}
}
For Cloud Firestore
https://firebase.google.com/docs/firestore/query-data/queries#compound_queries
Compound queries
You can chain multiple equality operators (== or array-contains) methods to create more specific queries (logical AND). However, you must create a composite index to combine equality operators with the inequality operators, <, <=, >, and !=.
citiesRef.where('state', '==', 'CO').where('name', '==', 'Denver');
citiesRef.where('state', '==', 'CA').where('population', '<', 1000000);
You can perform range (<, <=, >, >=) or not equals (!=) comparisons only on a single field, and you can include at most one array-contains or array-contains-any clause in a compound query:
Firebase doesn't allow querying with multiple conditions.
However, I did find a way around for this:
We need to download the initial filtered data from the database and store it in an array list.
Query query = databaseReference.orderByChild("genre").equalTo("comedy");
databaseReference.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
ArrayList<Movie> movies = new ArrayList<>();
for (DataSnapshot dataSnapshot1 : dataSnapshot.getChildren()) {
String lead = dataSnapshot1.child("lead").getValue(String.class);
String genre = dataSnapshot1.child("genre").getValue(String.class);
movie = new Movie(lead, genre);
movies.add(movie);
}
filterResults(movies, "Jack Nicholson");
}
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
}
});
Once we obtain the initial filtered data from the database, we need to do further filter in our backend.
public void filterResults(final List<Movie> list, final String genre) {
List<Movie> movies = new ArrayList<>();
movies = list.stream().filter(o -> o.getLead().equals(genre)).collect(Collectors.toList());
System.out.println(movies);
employees.forEach(movie -> System.out.println(movie.getFirstName()));
}
The data from firebase realtime database is as _InternalLinkedHashMap<dynamic, dynamic>.
You can also just convert this it to your map and query very easily.
For example, I have a chat app and I use realtime database to store the uid of the user and the bool value whether the user is online or not. As the picture below.
Now, I have a class RealtimeDatabase and a static method getAllUsersOnineStatus().
static getOnilineUsersUID() {
var dbRef = FirebaseDatabase.instance;
DatabaseReference reference = dbRef.reference().child("Online");
reference.once().then((value) {
Map<String, bool> map = Map<String, bool>.from(value.value);
List users = [];
map.forEach((key, value) {
if (value) {
users.add(key);
}
});
print(users);
});
}
It will print [NOraDTGaQSZbIEszidCujw1AEym2]
I am new to flutter If you know more please update the answer.
ref.orderByChild("lead").startAt("Jack Nicholson").endAt("Jack Nicholson").listner....
This will work.
Let's assume the following Grails domains:
Owner {
String name
static hasMany [cars: Cars]
}
Car {
Date inspectionDate
}
I want to be able to search for Owners through Criteria, with the following rule: Most recent Car by inspectionDate in Owner's cars list being lower than *given date*.
As an example, I want to apply the following code to a select query in GORM:
queryResultsList = allOwnersList.filter { owner ->
owner.cars.min{ car -> car.inspectionDate }.inspectionDate < myDate
}
I need to achieve it using Criteria because I am already filtering Owners on other fields.
The whole given code is used as an example, some parts of the original code has been ommited, and source code is not about cars and owners.
As on first thought I assumed I needed a subquery in SQL to retrieve my data as I expected, I tried the following:
Owner.createCriteria().list {
// [...] some filters on other fields
cars {
lt('inspectionDate', params.inspectionDate)
'in'('inspectionDate', new grails.gorm.DetachedCriteria(Owner).list {
projections {
cars {
min('inspectionDate')
}
}
})
}
}
I also tried to add groupProperty in different places in the projection, ending with MissingPropertyException.
I am using Grails 2.2.4
After a few days testing solutions, I've come to the following one:
Owner.createCriteria().list {
// [...] some filters on other fields
cars {
lt('inspectionDate', params.inspectionDate)
'in'('inspectionDate', Owner.createCriteria().list {
projections {
cars {
min('inspectionDate')
}
groupProperty 'id'
}
// this allows to only get first property, which is inspectionDate
}.collect { it[0] })
}
}
However, I still feel like doing things wrong. In particular, the collect part looks like a code smell.
I am posting this because it does what I need, but any help with finding a good solution will be appreciated.
I have the following code:
def customers = Customer.findAll()
def json = new JsonBuilder()
json {
customers.each { customer ->
id customer.id
name customer.name
address customer.address
}
}
I'm expecting that the result is an json array of customers, but instead it only contains 1 customer. Note the customers list contains 2 elements.
I saw some other post mentioning to use something like:
customers.collect {
Customer c -> [id: c.id, name: c.name, address: c.address]
}
But this style does not really fit nicely in the builder. E.g. I have to use colons : to assign values.
Is there another approach without creating groovy objects?
What you need is just this.
def json = new JsonBuilder( customers )
if there are no additional item in customer that is not required in the json.
No, json is a map, and you need a list of maps. What you are doing with each is re-assigning the id, name and address fields, so you just get the last value.
So you need to collect a list of maps together as you have in the question.