mysql - Simple many-to-many relationships -


apologies, haven't slept, want clarify in head.

a team can have many users, user can member of several teams.

is many-to-many relationship? if so, need third table join them in mysql?

many many relationships

many many relationships exist , you're right point works somehow differently many 1 / 1 many. it's not at all stupid question.

most of time, need add fields characteristics relation. because you'll want informations relationships themselves.

whether or not need information (and fields) determine if need third-part table or not.

real life example : men & women

let's have men , women tables. man can date many women through course of live, , reciprocally. so, have typical many many relationship. can without third table.

but now, suppose want add information: each relationship (be in romantic sense or in database sense (never thought sentence 1 day in life)), want tell when union started, , when ended.

then need third part table.

structure

let's have @ our real life example structure like.

i wrote in simplified doctrine2-inspired yaml, model runs symfony2 default:

tablename: man     fields:         id:             type: int             id: true         firstname:             type: string         dateofbirth:             type: datetime         favsport:             type: string         womenheprefers:             type: string     onetomany:         relationships:             target: manmeetswoman             mappedby: man             cascade: remove  tablename: woman     fields:         id:             type: int             id: true         firstname:             type: string         dateofbirth:             type: datetime         favauthor:             type: string         mensheprefers:             type: string     onetomany:         relationships:             target: manmeetswoman             mappedby: woman             cascade: remove  tablename: manmeetswoman     fields:         id:             type: int             id: true         dateofencounter:             type: datetime         dateofbreakup:             type: datetime     manytoone:         man:             target: man             inversedby: relationships         woman:             target: woman             inversedby: relationships

key points

  • your man , woman tables both have onetomany relationship manmeetswoman table
  • your manmeetswoman table have manytoone relationship both other tables
  • ideally, place cascading rules ensure when man deleted man table, relationships referencing him disappear third table.

what if don't want table

then have declare actual many many relationships.

it work pretty same, you won't have additional information existed between man , woman, other yeah, happened between them. don't know what, don't know when, don't know how long.

to this, replace onetomany , manytoone this:

# man:     manytomany:         woman:             target: woman  # woman:     manytomany:         man:             target: man

but begins tricky manage, without support of orm. so, third-part table remains best , easiest solution. plus, allows add informations relationship itself.


-

Comments

Post a Comment

Popular posts from this blog

html - Sizing a high-res image (~8MB) to display entirely in a small div (circular, diameter 100px) -

first of all, thank being patient know simple question more couple of weeks' experience figure out. i've seen folks persevere , helpful on here , hope you're willing me. here fiddle: http://jsfiddle.net/65uzf/ here 1 of places searched answers on here: ( vertically center image thumbnails fix-sized div ) tried solutions on there , didn't work out me. i've searched ~4 or 5 others, , been unsuccessful those, too. what want take large image (what linked background-image #ysypic , #tabpic divs) , display thumbnail version of image in confines of divs have set it. is, using background properties have laid out in css, portion of image displays inside of circular divs. without these properties, display defaults smaller section of image [presumably pi*(50^2) area in top left] terrible resolution. i not care if entire image doesn't display inside of circles (it won't, given difference in shape), , overflow not being displayed fine. want able see figu
Read more

java - IntelliJ - No such instance method -

using intellij's 12 ultimate, i'm running following code in debugger: java import play.api.libs.json.jsvalue; public class foo { ... public jsvalue tojson() { ... } public class fooexample { ... foo foo = new foo(); system.out.println("...); //<-- breakpoint at breakpoint, right-clicked source code , picked "evaluate expression," typing in: foo.tojson() . but following error showed up: no such instance method: play.api.libs.json.jsvalue$class.com.foo.foo.tojson () am doing wrong? foo#tojson calls scala code, if matters. edit had breakpoint after instantiation of foo. downvoted it, deserved it. edit my answer no longer valid after op modified question (i.e. moved breakpoint actual is).. a breakpoint hit before line executed. in case foo has not yet been declared or instantiated (i.e. foo constructor has not yet been called). you'll need put breakpoint @ next line (or step on current line) if want evaluate foo .
Read more

identifier - Is it possible for an html5 document to have two ids? -

i wondering if html5 element can have 2 ids, this: <button id="thing1" id="thing2" name="button1" onclick="change()">click here</button> or this: <button id="thing1, thing2" name="button1" onclick="change()">click here</button> also, if 1 of ids said 1 thing, , other id said thing, 1 used? tl;dr : no. in versions of html, each element can have 1 id. see the spec : the id attribute specifies element's unique identifier (id). [dom] the value must unique amongst ids in element's home subtree , must contain @ least 1 character. the value must not contain space characters. note: there no other restrictions on form id can take; in particular, ids can consist of digits, start digit, start underscore, consist of punctuation , etc. note: element's unique identifier can used variety of purposes, notably way link specific parts of document usin
Read more