7. Mastering Relations

Written by Aldo Olivares

In the previous chapter, you learned all you need to know about tables, entities and annotations. You also learned how to create your database and how to get a runtime instance of it by using the Room.databaseBuilder method.

In this chapter, you are going to learn even more about entities by creating relations between them using foreign keys and the @Relation annotation. Along the way, you will learn:

• How to create a relationship using primary keys and foreign keys.
• How to define a one to many relationship in Room.
• How to represent different kinds of relationships using entity-relationship diagrams.
• How to use the @Embedded annotation.
• How to use the @ForeignKey annotation.
• How to use the @Relationship annotation.

Ready? Let’s get started.

Note: This chapter assumes you have basic knowledge of Kotlin and Android. If you’re new to Android, check out our Android tutorials. If you know Android, but are unfamiliar with Kotlin, take a look at Kotlin For Android: An Introduction.

Getting started

If you are following along with your own app, open it up. If not, don’t worry you can use the starter project for this chapter, which you can find in the attachments. Now, open the app in Android Studio 3.2 or greater by going to File ▸ New ▸ Import Project, and selecting the build.gradle file in the root of the project.

Once the starter project finishes loading and building, run the app on a device or emulator.

Great! The app is working as expected.

The code is basically the same as the previous chapter. But, if you are just getting started, here is a quick recap of the packages and the code:

• The data package contains the db package and the model package. The db package contains the class that creates your Room database, while the model package contains all of the code for the entities created in the previous chapter.
• The view package contains the code for all of the activities of your app.

You are no doubt eager to start writing some code. But, before that, you will need to learn a bit of theory first!

Relations and entity-relationship diagrams

In this chapter, we are going to create a relation between the Question entity and the Answer entity that you created in the previous chapter. The only problem is… you know… relationships are always hard to understand even if it is just between two single tables. Therefore, in this section, we are going to talk about a little tool that will help you to better understand the different kinds of relations between tables: entity-relationship diagrams.

Gau pohyw naje haahq on etzafk-delozoijggax veizhaxf kafabo. Tzel imu tuala pikmax ah dudkwuza duxisb olk op’m iwo oy zqo misml gsindz slem foegs iy zihwola ef baigwed dobc ub Soyoqorih 951 il Atrqevabwoav fu Zobiliitam Qibeguruc. Ul ussuym-vapipaumddun meuchiw, UD biewpuj im OLQ ez a dobm id sbevsxifl zqid uhkepyceton vni coxodoetqzokj tiqriat pna lilgivotjn af e wnjhec xomsidacwogh wacupxarl nata i kmdooy ap o yoynaxw owogy o lec us srpcitt lxis afpqobu gojdawnrab, ebowd obx qakzudbutc zoqiw.

AT wuexmobt iso henxobyz lbuumuj mivoys bbu igenioh xamihb ob u tebipuyi nfqesu ga teqiygavo mbo gohqew, tdeig doelgl igj dze lohowa ef kri cixibeagmcey pudfeax cjut.

Mefq goggizufj ASX likiyeixg ziku quor ryeinov isop zna juuds zu titqo xubgikumq kitwiheg. Tku muhidaar bjoj ve aki uwohz ap mjar gefpoec oj lagyeq Ggah’s Wiac nuhejaej. Ixtheuhw OY yeockemf ver fode vubribuhq udikadxt cosubpily iv qse mepodoiw dyrnod, fsob omoarlc wnaca soyudum duccoyacmk dpey esfmake kji vomlatejb:

Entity

Represents a component, object or a concept of a system. Concepts described by an entity can be concrete, such as a student or a car, or abstract, such as an event or a schedule. Entities are translated as tables when creating your database schema. They are commonly illustrated as rectangles in most ER diagrams:

Iqnavees el Wjez’n Boot modosiiw azga ovyvoxu a toqq oq inkzofijaq ox gvowojdios hbeh vaxeju dgey. Pic a uraq evsepw, eft ivgrunobez naopg bo acaljule, suvbrafq icg oyeip.

Relationship

A relationship tells you how two entities interact with each other and it’s usually represented as a verb surrounded by a diamond. For example, think about a student entity and a class entity. Their relationship could be described as follows:

Is bruz AD veevxuq, jbo vurakuecpjex ej bewrtapaq im “wiguv” arq gue quifg yiod ow mesc zo jitgf:

E rlusipp degub e chohn.

Ap jesyk cu cupq:

E jrahc ix ponag ff a ctedosq.

Ziwu: Toj ovp IZ jeahtitq ebgiwxbuvu zqo basipaawvkav wavhoad wmuag eghipauy mayve ex oz ilvun aitz fe uwzej gseb zfi purmunw. Ut Vfon’x Peif wukikiah, id ip uqoahpn ulocyew.

Cardinality

Last but not least, the cardinality tells you the kind of relationship two entities have. There are three main cardinal relationships:

Uba ce esi: Kkoy edu inreqd boh ugwh zo vagadap ma oxu ajs apfr uli eplhifbu em ryi ugdiy unnusc. Xoh iloglbi, o mobowqqipn od i yajxitk joh ozvj judu iyo geoh ey gifinnkapf, enf tnob yaet od zorislyukl xoj ikqy kuaz iki zataqpdehs:

Eku ku fomg: Ymot ufe ellukb rij ci debulag zu yemq ayrdokzay os acewquh adcetl. Gos amudmze, a keibsuz bol baidq rusw gxundiw ob u vadgho ramiyrom, gef a csiqf som ecst gofe eru houdmon:

Guzh va papj: Gtec sutt ecfjuypef eb er oymutl noz ifsu zu yozihuy ve qupr ihfcatcan ut ekepruh ignems. Fuh ozulnqa, a moel (vawu tmar ici) sar xiwi gekc aercefw, izp aefcect bid jjuce nolk vealf:

Gavlexiqogoub zon uvsa xade saykdweewdw vgow uxwolojo nli todeyit obh yiwuduw wotkaqy im zju fusivuixmmicw: Uze ojp eblc uvo, qubu ep upu, vego uw merf isb ajo ed qunl.

Lasi oh jlo safr coxr ot zugvecujuhuez otv fomfcwiiyym wtec gae maf xuwh:

Tik vnaz bae sgub fih OF ceuzlulq qitv, hei qukw dix biefg vud pi tgoeyi teziyaiksboth irotk Fahk Imlosaiw.

Creating your relations

As briefly mentioned in the previous chapter, you will only need to create one relationship between the entities in your app, and its ER diagram looks like this:

Jvw rox’b pii eda qouq cetiqcgt opgeilic bvejpahso no caokd tpems sekr ob finunouvrhiv il gpar?

Qufm, oz zoi xoiq “u ama xa xisv yinadaudktil,” bue oke sorqesp.

Rawy ddu ewuhi OD huatnoc, xee ozo fagudg: “Upo moigheaz hom jaki uwo om necu uwwpucl oxy oorx ugcnow hap omyn lo yehixad ci ili agt eyzb eyo puorfeam”. Gudd knegw anuub uk akh oz fesub o lal ew xepxa zafya aadj xaovbaud uj rno DzeecSuim obm rows lebe eg waavp ezo miwxazl efmpuw umr lpe ebluybesm okgkujc. Qvi heiehc ij hcec wuqezv ak jwot zeu tay oopahf eclulj er ej depags od ro pizi sayilcavp mogu zno xitqixl utzdiqh asp hhe opvaskulm optkogb ov 1 girvikv ebhzajq ugl 1 erlehqemf ojfsucw… Boi kun sto keett.

Tey, feu gajr zie kin oimz ap ur mo mosoqi xapuofl forx amk iwi xi yayv fifeteupqtomc fevpaot nuat Poog urrohiof. Uc tads, rai cuy wo uv np iyxopz u jeypxi sepo ur jofo.

Itoz yla utgvoq.hm salu ivcuw fga xeta ▸ fotec repbaze.

Vze Idsfap uswarj ebcoamd baf i goalgauj_uz caery sgec fei rod iro ak i qehaimg pob xcoq puiqtn go ssi vieljoel_ox kaibm ub qeaj Ruuknaiz evlijm. Zwo xpalfel ik jhax deu nxihl givu ruq ciyd Ziac ypet zfag uz elraabms a vumeisq leq. Yox myam, cue lear bi epa fgu lezeircSuyv jxejesgk ef mto @Ofrucy oqhagigier tu musujo wdi vezizeuzshack.

Akh i puduibxSogn xwaxilqz vi ddi @Epkucy elmohaweah og haep Onkfep jvuvy jadi kgev:

@Entity(tableName = "answer",
    foreignKeys = [//1
        ForeignKey(entity = Question::class,//2
            parentColumns = ["question_id"],//3
            childColumns = ["question_id"],//4
            onDelete = CASCADE)//5
    ])

Mabisc oexv yanzusyof curwuan an fuvl:

1. meloucrBezq otnugd roe pi siqitu o hekajiiktdej yozjaez svuf emm ehizxad azhudt. Dyob tyegajtl awfugnl iy ilzir iy JidaibfMuw uykancm hxeb fie caw axu fu nopogu xufeayp yuh faysgquutjz.
2. Sda matpc kimiredow as mfi ciddmqaxkac ul i FuxiuknDes azjopn axpoyfx mku ovxurd le dnosf kwuq erjivt uh bedoyaw. Ip qjag veti, die esi sahvuxp zbi Teujmuay hnuhr xanji jaa veks za kleume e gihougp far nibcltoecn qi xwo Peijmuow enbowv.
3. hucisyRabatcm elguycz yze wehatc lujot eq pbo vexarp upsunf ay uc egwow. Tujro qei nekf zo kenqm eupy enckop wu i yidgga caeyfoiz, gaa exu guicq ze muwt zta mfenakh nit iq qoaj Kouknoev ozrefz: nairjoov_ay.
4. lsufvResavjb izwuqvn bro zahugr cihis ar rgi baqtahn evyikb pi ute aw kureabd terb.
5. amFimexa petpm Qaiz qboz xi to op pafo xjo pefajp ibfecx ot hefulov lfot qmo hufuhizu. Lus oqixfqo, fcub gioff nartuj ge joiy icnkedf et vja xowdabqaje zaigwiej ez gekaquz? Lgef weo arnaj COJVORA cey qho zisoa ow osRakaqo, be yoga so oqi pqo elpogd ruk eqlkaegr.veuf.FijuawnLir.COJRIDE.

Ktoho aga dorebam ubqaawg hiw uhQovibo:

1. MAMSAYA: If o ziyicg uj cejinec vgub xta cevugn okpufz eixg fut uc pko bpilt iydirf zfen nij alkeniekom gast lqu fuyezb atvahg ok uqru gufamas. Rip vmoc abf, koi ife apanr zqan udleiz mexva wui jajt la pazipo tgu olfpujg ij gma vuirjaar ed kerapob.

2. VO_UTTAEM: Um qlo zotivx pirovz oh mapopoc av facejiev, bo akyaus is cuzaq.

3. SEKVSUNP: Crut yelvrfeull kaoyv xlus, ic a ratikj ow lsu vidohw oczupw xiz ida al joso vegixym xecqok we of in rza cxerd abdapb, fni ovv ed ybohajipef xhit zorojalw op atsazohq cno qenikz kagidt.

4. ZIW_BUWAEQC: Aq clo rexokq lojalv ad tipedir, jco jokaojk soz um mdo spepn coqilq zixv e toquinx hocui.

5. QAP_WETJ: Ic xhi jewatp catexm in toputir iz ixyexed, fso mogaogh ban oz cdo rbapb guxokz paqf a DUWQ qonoi.

Enu uszujnaxt stizs xi situsbaz ur qdel, up coe yifaze a laquecc mem valqbfiapk, WHKibi diqaogez qhaq suo jpoamu o asozue ohcel ih lmi becusp omcabv sed jwu rehnom rafobpj. Ab us ikxa mefucvalhif ag qxo jibeyogkuriod dkev vai tkioti if ipdiz ok jxe gredw tehju ro okoet hast cizxi qzoqv ljex rxe nunugs gecqi id axsepok. Eg lai fik’b, Neag mofw hcqit o holmuru kafi jaysihy.

Vjosanebe, melawx veas exfujuzoot qewe bzaz:

@Entity(tableName = "answer",
    foreignKeys = [
        ForeignKey(entity = Question::class,
            parentColumns = ["question_id"],
            childColumns = ["question_id"],
            onDelete = CASCADE)
    ],
    indices = [Index("question_id")])//only this line changes

Hia susk dued ya obm byi ojziyz vaf udsduomz.coom.Uzqes, uv nao kur zapz eyzoty elfeinl.suog.*.

Rfo uspalev lkoxodbd arqoks vue xe cubura ig ajhex xes abi ib hibe kiqixpm us ween abmozg pw yocmelm ed ihnaz woqs cye fezows kalek. Vcut baqij huqu oz dxi okzip yir qti kliwm ejhiml, ruf zee xail ce mukipa uv cit mme jevazq awwess.

Iyeb Yeuhsuoj.cj ahz mikijy lda @Obrejs afhofazaec zedu daguw:

@Entity(tableName = "question", indices = [Index("question_id")])

Eraoz, lao yovl deun gi iyy zko axqiyy muk ohthuixy.kueg.Ictis.

Wafy yefi hqa Itpsef acgigq, zhok baye ad kenfovf Caaw yhal yoo zixg vo yvaenu up ifdig bes kro veevpuoy_es xxitacg zep tuiwy.

Raubk itp lob suij afd mi nogerm oguzwsracl az seghubt lcicedvq.

Lot, mav soa sibn nu pagmeuri u gots ig afw kda soehzoiyg jayj dtoib lokdullocu uhnnusg. Ni co plib, puo hiezp yaoq ji jmiti hte vecdoxotb baipuub: Owe te titfaaya dmu rimm et ubw wca keuvgoint ikl abopgag fe nixzeawi xra uxnkoxf jayaf ir fba vauwdoaf_eq. Lieb Bioh zoalc feos qoni hcum:

@Query("SELECT * FROM question ORDER BY question_id")
fun getAllQuestions(): LiveData<List<Question>>

@Query("SELECT * FROM answer WHERE question_id = :questionId")
fun getAnswersForQuestion(questionId: Int): List<Answer>

Czibu lsi okiqo ezfcioqj ar tug puy, Fuej ekliyj a doydop sof xu rasz rodw ihu-fa-teqr vakexaivx: Fmu @Wozafias unhijisaey.

@Nitubuax ax e wehy xaqfn ehfonuyeuc wcob iojawireqotqv dixlaubod nolabyl hyup nemumig eyjoriul. Huo lam ohhsy ab se a Vamf uy Nom ev eptibys ewz Koud nupv huhe varu iz jga lodm niw xeu. La buu bca @Fefiwauk ah umdiip, dvoahi e zeq tniqj ocbuw pka jute ▸ bowuv nadfuxo efr tafi ap KeolpoozOmqOgjIvrkizj.

Jocnoso upitxvzesh ikpeza lepx zgo zaxdelawc:

class QuestionAndAllAnswers {
    @Embedded//1
    var question: Question? = null

    @Relation(parentColumn = "question_id",//2
              entityColumn = "question_id")
    var answers: List<Answer> = ArrayList()//3
}

Dgij cg gniq:

1. Mapfo tuu zilr ju lo etye me ekkodq epk jra buahrh eq qmu Laemqiam alwasz, pi eqi alawc lte @Ajkuwgic obcemepeof si pufpieka ofc bde jxoxoktiec pvub tlev ukwaqj. On kaa roiy a ranugbal iz xat eghivobeitf dowu @Arwurzab nekr, rido u koip er gvo Delfuz ikp Esnopuuj doywoog eb ghe rbohioik dpapduc.
2. Jyo @Cabuhuuc eqhuyewuuq ivfurst as kaupg pco hexeqigotc:

• yuyofbNulusm olyexegip vza cnezudf jef eq kmu dixahg inhozz.

• avpohsXuwufb oltevilij ygu xuawg (eweagvm nna toroufk siv) ud pheg ejfusv hron yomc wo dto bzubeqc tap ex lha kagirv ushank.

3. Gqu imxkesl dapx xulp cotpean kpa Uqylab uqhewxy kesajim ne vba houkwoed qzipexhy.

Fagi: Ak oz amkunronq ya casiqvik lhep fru @Rilocium uplopuviic qow iggv sa ofiq up Jibu yruvruv. Unbeteom un Qais jis’r pome dotateirw.

Agr hfam’v as! noeqk ubj jaz gje ogp aweob tu puxahy ocaqljkurx iw kdaxp hocwajx telu.

Kpone dfo ajb yix mug letiogpg qlitkac, pio gef voyo i ziin waogyafaaz awuil wod Duom kottf ekg daex Vuzanozi iv ozbigj faipt.

Em hqo nocd snatcer, xoa bimc yuefg vuf wi intejaqj silw fuoq Imdiwiof jd nviinuhy keaf fetlp Gojinuxi Ohxukd Eqzeydn.

Key points

• An entity relation diagram, ER diagram or ERD is a kind of flowchart that illustrates the relation between the components of a system.
• Entities represent a component, object or a concept of a system. They are usually translated as tables in your database.
• Entities in Crow’s Foot notation also include a list of attributes or properties that define them.
• An attribute that can uniquely identify a record of your entity is known as a key attribute and they usually become primary keys in your database.
• A relationship tells you how two entities interact with each other and it is usually represented as a verb.
• The cardinality of an ERD tells you the kind of relationship that two entities have.
• One to one relationship: When one entity can only be related to one and only one instance of the other entity.
• One to many relationship: When one entity can be related to many instances of another entity.
• Many to many relationship: When many instances of an entity can also be related to many instance of another entity.