Firebase how to index this data to be fetched? -


i have db users , items.

every user has languages, array of languages, example ['en', 'ar']

every item has language, string, example 'en'.

how can index items, such can list of last x items in array of languages? (i.e - latest 5 items either 'en' or 'ar')

for single language solution simple - have index has language key, , array of item keys ordered whatever.

please note firebase official documentation recommends against using arrays. imho, main problem in project trying use array, anti-pattern when comes firebase. beside that, 1 of many reasons firebase recommends against using arrays makes security rules impossible write. because firebase database structured pairs of key , values, best option use map.

to achieve want, recomand using database structure looks this:

firebase-root     |     --- users     |     |     |     --- uid1     |     |    |     |     |    --- languages     |     |            |     |     |            --- languageid1: true     |     |            |     |     |            --- languageid2: true     |     |     |     --- uid2     |          |     |          --- languages     |                  |     |                  --- languageid3: true     |                  |     |                  --- languageid4: true     |     --- languages     |      |     |      --- languageid1     |      |      |     |      |      --- languagename: "en"     |      |      |     |      |      --- items     |      |           |     |      |           --- itemid1: true     |      |     |      --- languageid2: "ar"     |      |      |     |      |      --- languagename: "ar"     |      |      |     |      |      --- items     |      |           |     |      |           --- itemid2: true     |     --- item          |          --- itemid1          |     |          |     --- title: "itemtitleen"          |     |          |     --- en: true          |          --- itemid2                |                --- title: "itemtitlear"                |                --- ar: true

now, database structure can achieve want. example, can query database display languages database. can display languages of single user.

if want query database last x item have language set en, need put listener on items node , create query using functions like: orderbychild(), equalsto() , limittolast(). such query should this:

query = rootref.child("items")     .orderbychild("en")     .equalsto(true)     .limittolast(5)     .addlistener(/* ... */)

edit: unfortunately firebase not allow multiple conditions in query. in firebase there no clause sounds this: where language = "en" , language = "ar". solve this, need put listener on other node, on languages node.

the flow follows:

yourref = rootref.child("languages"); //only 1 listener yourref.addlistener(     1. create list     2. items datasnapshot.child("languageid1").child("items").getchildren()     3. add **en** items list     4. items datasnapshot.child("languageid2").child("items").getchildren()     5. add **ar** items list     6. display list contains data both languages )

hope helps.


Comments

Post a Comment

Popular posts from this blog

python - Operations inside variables -

this question has answer here: evaluating mathematical expression in string 12 answers i want make python script finds maximum possible output using 2 operations. problem says there syntax error @ 2nd last line, in b in there. how can fix it? x = 0 = int(input("1st number:")) c = int(input("2nd number:" )) e = int(input("3rd number:" )) in range(4): if == 0: b = "+" elif == 1: b = "-" elif == 2: b = "/" else: b = "*" j in range(4): if j == 0: d = "+" elif j == 1: d = "-" elif j == 2: d = "/" else: d = "*" k = b c d e print(k) if you're after variables e on single line, couldn...
Read more

Generic Map Parameter java -

i having lot of methods of type: public static list<eduusers> getdetails(class c, map<string, integer> params) { list<eduusers> details = null; session session = hibernateutil.getsessionfactory().opensession(); transaction tx = null; try { tx = session.begintransaction(); criteria cr = session.createcriteria(c); (map.entry<string, integer> entry : params.entryset()) { cr.add(restrictions.eq(entry.getkey(), entry.getvalue())); } details = cr.list(); tx.commit(); } catch (exception asd) { log.debug(asd.getmessage()); if (tx != null) { tx.commit(); } } { session.close(); } return details; } i trying have generic method them , have written far: public static <t> list<t> getdetails(class<t> c, class<t> b) { l...
Read more

arrays - What causes a java.lang.ArrayIndexOutOfBoundsException and how do I prevent it? -

what arrayindexoutofboundsexception mean , how rid of it? here code sample triggers exception: string[] name = {"tom", "dick", "harry"}; for(int = 0; i<=name.length; i++) { system.out.print(name[i] +'\n'); } your first port of call should documentation explains reasonably clearly: thrown indicate array has been accessed illegal index. index either negative or greater or equal size of array. so example: int[] array = new int[5]; int boom = array[10]; // throws exception as how avoid it... um, don't that. careful array indexes. one problem people run thinking arrays 1-indexed, e.g. int[] array = new int[5]; // ... populate array here ... (int index = 1; index <= array.length; index++) { system.out.println(array[index]); } that miss out first element (index 0) , throw exception when index 5. valid indexes here 0-4 inclusive. correct, idiomatic for statement here be: for (int index = 0; index <...
Read more