{"title":"Nathanna Coinníollach","version":"5.2","locale":"ga","docname":"ref/models/conditional-expressions","url":"/ga/5.2/ref/models/conditional-expressions/","canonical":"https://djangodocs.dev/ga/5.2/ref/models/conditional-expressions/","summary":"Ligeann abairtí coinníollach duit úsáid:eochairfhocal: if `... :keyword: `elif... :keyword: loighic else laistigh de scagairí, anótaí, comhiomlánaithe agus…","html":"

Nathanna CoinníollachLink to this heading#

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Ligeann abairtí coinníollach duit úsáid:eochairfhocal: if `... :keyword: `elif... :keyword: loighic else laistigh de scagairí, anótaí, comhiomlánaithe agus nuashonruithe. Déanann abairt choinníollach measúnú ar shraith coinníollacha do gach sraith de thábla agus tugann sé an léiriú torthaí meaitseála ar ais. <expressions>Is féidir nathanna coinníollach a chomhcheangal agus a neadadh cosúil le chéile freisin: doc: `abairtí `.

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Na ranganna léirithe coinníollachLink to this heading#

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Beidh muid ag úsáid an tsamhail seo a leanas sna samplaí ina dhiaidh sin:

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Code
from django.db import models\n\n\nclass Client(models.Model):\n    REGULAR = "R"\n    GOLD = "G"\n    PLATINUM = "P"\n    ACCOUNT_TYPE_CHOICES = {\n        REGULAR: "Regular",\n        GOLD: "Gold",\n        PLATINUM: "Platinum",\n    }\n    name = models.CharField(max_length=50)\n    registered_on = models.DateField()\n    account_type = models.CharField(\n        max_length=1,\n        choices=ACCOUNT_TYPE_CHOICES,\n        default=REGULAR,\n    )\n
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Cén uaireLink to this heading#

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\nclass When(condition=None, then=None, **lookups)Link to this definition#
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Úsáidtear réad Nuair () ``chun coinníoll agus a thoradh a chumhdú le húsáid sa léiriú coinníollach. Tá réad ``When () ``a úsáid cosúil leis an modh: meth: `~django.db.models.query.queryset.filter` a úsáid. Is féidir an coinníoll a shonrú ag baint úsáidea:ref: `field lookups <field-lookups>`, :class: `~django.db.models.q` rudaí, nó: class: `~django.db.Models.Expression` rudaí a bhfuil ``output_field atá a:class: ~django.db.models.booleanfield. Cuirtear an toradh ar fáil ag baint úsáide as an eochairfhocal ``then ``.

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Roinnt samplaí:

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Python console
>>> from django.db.models import F, Q, When\n>>> # String arguments refer to fields; the following two examples are equivalent:\n>>> When(account_type=Client.GOLD, then="name")\n>>> When(account_type=Client.GOLD, then=F("name"))\n>>> # You can use field lookups in the condition\n>>> from datetime import date\n>>> When(\n...     registered_on__gt=date(2014, 1, 1),\n...     registered_on__lt=date(2015, 1, 1),\n...     then="account_type",\n... )\n>>> # Complex conditions can be created using Q objects\n>>> When(Q(name__startswith="John") | Q(name__startswith="Paul"), then="name")\n>>> # Condition can be created using boolean expressions.\n>>> from django.db.models import Exists, OuterRef\n>>> non_unique_account_type = (\n...     Client.objects.filter(\n...         account_type=OuterRef("account_type"),\n...     )\n...     .exclude(pk=OuterRef("pk"))\n...     .values("pk")\n... )\n>>> When(Exists(non_unique_account_type), then=Value("non unique"))\n>>> # Condition can be created using lookup expressions.\n>>> from django.db.models.lookups import GreaterThan, LessThan\n>>> When(\n...     GreaterThan(F("registered_on"), date(2014, 1, 1))\n...     & LessThan(F("registered_on"), date(2015, 1, 1)),\n...     then="account_type",\n... )\n
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Coinnigh i gcuimhne gur féidir le gach ceann de na luachanna seo a bheith ina léiriú.

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``Cás ``Link to this heading#

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\nclass Case(*cases, **extra)Link to this definition#
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Tá abairt Cás () ``cosúil leis an:keyword: `if `... :keyword: `elif`... :keyword: ráiteas `else` i ``Python. Déantar gach coinníoll sna rudaí Nuair () ``curtha ar fáil a mheas in ord, go dtí go ndéanfaidh duine measúnú ar luach fírinneach. Cuirtear an abairt ``torth ón réad meaitseála ``When () ``ar ais.

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Sampla:

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Python console
>>>\n>>> from datetime import date, timedelta\n>>> from django.db.models import Case, Value, When\n>>> Client.objects.create(\n...     name="Jane Doe",\n...     account_type=Client.REGULAR,\n...     registered_on=date.today() - timedelta(days=36),\n... )\n>>> Client.objects.create(\n...     name="James Smith",\n...     account_type=Client.GOLD,\n...     registered_on=date.today() - timedelta(days=5),\n... )\n>>> Client.objects.create(\n...     name="Jack Black",\n...     account_type=Client.PLATINUM,\n...     registered_on=date.today() - timedelta(days=10 * 365),\n... )\n>>> # Get the discount for each Client based on the account type\n>>> Client.objects.annotate(\n...     discount=Case(\n...         When(account_type=Client.GOLD, then=Value("5%")),\n...         When(account_type=Client.PLATINUM, then=Value("10%")),\n...         default=Value("0%"),\n...     ),\n... ).values_list("name", "discount")\n<QuerySet [('Jane Doe', '0%'), ('James Smith', '5%'), ('Jack Black', '10%')]>\n
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Glacann Cás () ``le haon líon de chrudaí ``Nuair () mar argóintí aonair. Cuirtear roghanna eile ar fáil trí argóintí eochairfhocal Mura ndéanann aon cheann de na coinníollacha measúnú ar TRUE, ansin cuirtear an abairt a thugtar leis an argóint eochairfhocal réamhshocraithe ar ais. Mura gcuirtear argóint réamhshocraithe ar fáil, úsáidtear None.

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Dá mbeimis ag iarraidh ár bhfiosrú roimhe seo a athrú chun an lascaine a fháil bunaithe ar cé chomh fada agus a bhí an `Cliant ``linn, d'fhéadfaimis é sin a dhéanamh ag baint úsáide as cuardaigh:

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Python console
>>> a_month_ago = date.today() - timedelta(days=30)\n>>> a_year_ago = date.today() - timedelta(days=365)\n>>> # Get the discount for each Client based on the registration date\n>>> Client.objects.annotate(\n...     discount=Case(\n...         When(registered_on__lte=a_year_ago, then=Value("10%")),\n...         When(registered_on__lte=a_month_ago, then=Value("5%")),\n...         default=Value("0%"),\n...     )\n... ).values_list("name", "discount")\n<QuerySet [('Jane Doe', '5%'), ('James Smith', '0%'), ('Jack Black', '10%')]>\n
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Oibríonn Cás () ``freisin i gclásal ``scagaire (). Mar shampla, chun cliaint óir a chláraigh níos mó ná mí ó shin agus cliaint platanam a chláraigh níos mó ná bliain ó shin a fháil:

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Python console
>>> a_month_ago = date.today() - timedelta(days=30)\n>>> a_year_ago = date.today() - timedelta(days=365)\n>>> Client.objects.filter(\n...     registered_on__lte=Case(\n...         When(account_type=Client.GOLD, then=a_month_ago),\n...         When(account_type=Client.PLATINUM, then=a_year_ago),\n...     ),\n... ).values_list("name", "account_type")\n<QuerySet [('Jack Black', 'P')]>\n
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Fiosruithe chun cinnLink to this heading#

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Is féidir nathanna coinníollach a úsáid in anótaí, comhiomlánaithe, scagairí, cuardaigh agus nuashonruithe. Is féidir iad a chomhcheangal agus a neadú le nathanna eile freisin. Ligeann sé seo duit ceisteanna coinníollacha cumhachtacha a dhéanamh.

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Nuashonrú coinníollLink to this heading#

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Abair go dteastaíonn uainn an account_type` a athrú chun dátaí cláraithe a mheaitseáil dátaí cláraithe. Is féidir linn é seo a dhéanamh ag baint úsáide as abairt choinníollach agus an modh:meth: ~django.db.models.query.queryset.update:

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Python console
>>> a_month_ago = date.today() - timedelta(days=30)\n>>> a_year_ago = date.today() - timedelta(days=365)\n>>> # Update the account_type for each Client from the registration date\n>>> Client.objects.update(\n...     account_type=Case(\n...         When(registered_on__lte=a_year_ago, then=Value(Client.PLATINUM)),\n...         When(registered_on__lte=a_month_ago, then=Value(Client.GOLD)),\n...         default=Value(Client.REGULAR),\n...     ),\n... )\n>>> Client.objects.values_list("name", "account_type")\n<QuerySet [('Jane Doe', 'G'), ('James Smith', 'R'), ('Jack Black', 'P')]>\n
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Comhiomlánú coinníollachLink to this heading#

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Cad a tharlaíonn más mian linn a fháil amach cé mhéad cliant atá ann do gach account_type? <aggregation-functions>Is féidir linn an argóint scagair de:ref: feidhmeanna comhiomlána` a úsáid chun é seo a bhaint amach:

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Python console
>>> # Create some more Clients first so we can have something to count\n>>> Client.objects.create(\n...     name="Jean Grey", account_type=Client.REGULAR, registered_on=date.today()\n... )\n>>> Client.objects.create(\n...     name="James Bond", account_type=Client.PLATINUM, registered_on=date.today()\n... )\n>>> Client.objects.create(\n...     name="Jane Porter", account_type=Client.PLATINUM, registered_on=date.today()\n... )\n>>> # Get counts for each value of account_type\n>>> from django.db.models import Count\n>>> Client.objects.aggregate(\n...     regular=Count("pk", filter=Q(account_type=Client.REGULAR)),\n...     gold=Count("pk", filter=Q(account_type=Client.GOLD)),\n...     platinum=Count("pk", filter=Q(account_type=Client.PLATINUM)),\n... )\n{'regular': 2, 'gold': 1, 'platinum': 3}\n
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Táirgeann an comhiomlán seo ceist leis an gcomhréir SQL 2003 FILTER WHERE ar bhunachair sonraí a thacaíonn leis:

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SQL
SELECT count('id') FILTER (WHERE account_type=1) as regular,\n       count('id') FILTER (WHERE account_type=2) as gold,\n       count('id') FILTER (WHERE account_type=3) as platinum\nFROM clients;\n
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Ar bhunachair sonraí eile, déantar aithris a dhéanamh air seo ag baint úsáide as ráiteas CÁISE:

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SQL
SELECT count(CASE WHEN account_type=1 THEN id ELSE null) as regular,\n       count(CASE WHEN account_type=2 THEN id ELSE null) as gold,\n       count(CASE WHEN account_type=3 THEN id ELSE null) as platinum\nFROM clients;\n
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Tá an dá ráiteas SQL coibhéiseach go feidhmiúil ach b'fhéidir go bhfeidhmeoidh an FILTER` níos follasaí feidhmiú níos fearr.

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Scagaire coinníollachLink to this heading#

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Nuair a thugann abairt choinníollach luach boolean ar ais, is féidir é a úsáid go díreach i scagairí. Ciallaíonn sé seo nach gcuirfear é leis na colúin SELECT, ach is féidir leat é a úsáid fós chun torthaí a scagadh:

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Python console
>>> non_unique_account_type = (\n...     Client.objects.filter(\n...         account_type=OuterRef("account_type"),\n...     )\n...     .exclude(pk=OuterRef("pk"))\n...     .values("pk")\n... )\n>>> Client.objects.filter(~Exists(non_unique_account_type))\n
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I dtéarmaí SQL, déanann sé meastóireacht ar:

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SQL
SELECT ...\nFROM client c0\nWHERE NOT EXISTS (\n  SELECT c1.id\n  FROM client c1\n  WHERE c1.account_type = c0.account_type AND NOT c1.id = c0.id\n)\n
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