1 """

     2 5. Many-to-many relationships

     3 

     4 To define a many-to-many relationship, use ManyToManyField().

     5 

     6 In this example, an article can be published in multiple publications,

     7 and a publication has multiple articles.

     8 """

     9 

    10 from django.db import models

    11 

    12 class Publication(models.Model):

    13     title = models.CharField(maxlength=30)

    14 

    15     def __str__(self):

    16         return self.title

    17 

    18     class Meta:

    19         ordering = ('title',)

    20 

    21 class Article(models.Model):

    22     headline = models.CharField(maxlength=100)

    23     publications = models.ManyToManyField(Publication)

    24 

    25     def __str__(self):

    26         return self.headline

    27 

    28     class Meta:

    29         ordering = ('headline',)

    30 

    31 __test__ = {'API_TESTS':"""

    32 # Create a couple of Publications.

    33 >>> p1 = Publication(id=None, title='The Python Journal')

    34 >>> p1.save()

    35 >>> p2 = Publication(id=None, title='Science News')

    36 >>> p2.save()

    37 >>> p3 = Publication(id=None, title='Science Weekly')

    38 >>> p3.save()

    39 

    40 # Create an Article.

    41 >>> a1 = Article(id=None, headline='Django lets you build Web apps easily')

    42 >>> a1.save()

    43 

    44 # Associate the Article with a Publication.

    45 >>> a1.publications.add(p1)

    46 

    47 # Create another Article, and set it to appear in both Publications.

    48 >>> a2 = Article(id=None, headline='NASA uses Python')

    49 >>> a2.save()

    50 >>> a2.publications.add(p1, p2)

    51 >>> a2.publications.add(p3)

    52 

    53 # Adding a second time is OK

    54 >>> a2.publications.add(p3)

    55 

    56 # Add a Publication directly via publications.add by using keyword arguments.

    57 >>> new_publication = a2.publications.create(title='Highlights for Children')

    58 

    59 # Article objects have access to their related Publication objects.

    60 >>> a1.publications.all()

    61 [<Publication: The Python Journal>]

    62 >>> a2.publications.all()

    63 [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]

    64 

    65 # Publication objects have access to their related Article objects.

    66 >>> p2.article_set.all()

    67 [<Article: NASA uses Python>]

    68 >>> p1.article_set.all()

    69 [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]

    70 >>> Publication.objects.get(id=4).article_set.all()

    71 [<Article: NASA uses Python>]

    72 

    73 # We can perform kwarg queries across m2m relationships

    74 >>> Article.objects.filter(publications__id__exact=1)

    75 [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]

    76 >>> Article.objects.filter(publications__pk=1)

    77 [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]

    78 >>> Article.objects.filter(publications=1)

    79 [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]

    80 >>> Article.objects.filter(publications=p1)

    81 [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]

    82 

    83 >>> Article.objects.filter(publications__title__startswith="Science")

    84 [<Article: NASA uses Python>, <Article: NASA uses Python>]

    85 

    86 >>> Article.objects.filter(publications__title__startswith="Science").distinct()

    87 [<Article: NASA uses Python>]

    88 

    89 # The count() function respects distinct() as well.

    90 >>> Article.objects.filter(publications__title__startswith="Science").count()

    91 2

    92 

    93 >>> Article.objects.filter(publications__title__startswith="Science").distinct().count()

    94 1

    95 

    96 >>> Article.objects.filter(publications__in=[1,2]).distinct()

    97 [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]

    98 >>> Article.objects.filter(publications__in=[1,p2]).distinct()

    99 [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]

   100 >>> Article.objects.filter(publications__in=[p1,p2]).distinct()

   101 [<Article: Django lets you build Web apps easily>, <Article: NASA uses Python>]

   102 

   103 # Reverse m2m queries are supported (i.e., starting at the table that doesn't

   104 # have a ManyToManyField).

   105 >>> Publication.objects.filter(id__exact=1)

   106 [<Publication: The Python Journal>]

   107 >>> Publication.objects.filter(pk=1)

   108 [<Publication: The Python Journal>]

   109 

   110 >>> Publication.objects.filter(article__headline__startswith="NASA")

   111 [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]

   112 

   113 >>> Publication.objects.filter(article__id__exact=1)

   114 [<Publication: The Python Journal>]

   115 >>> Publication.objects.filter(article__pk=1)

   116 [<Publication: The Python Journal>]

   117 >>> Publication.objects.filter(article=1)

   118 [<Publication: The Python Journal>]

   119 >>> Publication.objects.filter(article=a1)

   120 [<Publication: The Python Journal>]

   121 

   122 >>> Publication.objects.filter(article__in=[1,2]).distinct()

   123 [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]

   124 >>> Publication.objects.filter(article__in=[1,a2]).distinct()

   125 [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]

   126 >>> Publication.objects.filter(article__in=[a1,a2]).distinct()

   127 [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>, <Publication: The Python Journal>]

   128 

   129 # If we delete a Publication, its Articles won't be able to access it.

   130 >>> p1.delete()

   131 >>> Publication.objects.all()

   132 [<Publication: Highlights for Children>, <Publication: Science News>, <Publication: Science Weekly>]

   133 >>> a1 = Article.objects.get(pk=1)

   134 >>> a1.publications.all()

   135 []

   136 

   137 # If we delete an Article, its Publications won't be able to access it.

   138 >>> a2.delete()

   139 >>> Article.objects.all()

   140 [<Article: Django lets you build Web apps easily>]

   141 >>> p2.article_set.all()

   142 []

   143 

   144 # Adding via the 'other' end of an m2m

   145 >>> a4 = Article(headline='NASA finds intelligent life on Earth')

   146 >>> a4.save()

   147 >>> p2.article_set.add(a4)

   148 >>> p2.article_set.all()

   149 [<Article: NASA finds intelligent life on Earth>]

   150 >>> a4.publications.all()

   151 [<Publication: Science News>]

   152 

   153 # Adding via the other end using keywords

   154 >>> new_article = p2.article_set.create(headline='Oxygen-free diet works wonders')

   155 >>> p2.article_set.all()

   156 [<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]

   157 >>> a5 = p2.article_set.all()[1]

   158 >>> a5.publications.all()

   159 [<Publication: Science News>]

   160 

   161 # Removing publication from an article:

   162 >>> a4.publications.remove(p2)

   163 >>> p2.article_set.all()

   164 [<Article: Oxygen-free diet works wonders>]

   165 >>> a4.publications.all()

   166 []

   167 

   168 # And from the other end

   169 >>> p2.article_set.remove(a5)

   170 >>> p2.article_set.all()

   171 []

   172 >>> a5.publications.all()

   173 []

   174 

   175 # Relation sets can be assigned. Assignment clears any existing set members

   176 >>> p2.article_set = [a4, a5]

   177 >>> p2.article_set.all()

   178 [<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]

   179 >>> a4.publications.all()

   180 [<Publication: Science News>]

   181 >>> a4.publications = [p3]

   182 >>> p2.article_set.all()

   183 [<Article: Oxygen-free diet works wonders>]

   184 >>> a4.publications.all()

   185 [<Publication: Science Weekly>]

   186 

   187 # Relation sets can be cleared:

   188 >>> p2.article_set.clear()

   189 >>> p2.article_set.all()

   190 []

   191 >>> a4.publications.all()

   192 [<Publication: Science Weekly>]

   193 

   194 # And you can clear from the other end

   195 >>> p2.article_set.add(a4, a5)

   196 >>> p2.article_set.all()

   197 [<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]

   198 >>> a4.publications.all()

   199 [<Publication: Science News>, <Publication: Science Weekly>]

   200 >>> a4.publications.clear()

   201 >>> a4.publications.all()

   202 []

   203 >>> p2.article_set.all()

   204 [<Article: Oxygen-free diet works wonders>]

   205 

   206 # Relation sets can also be set using primary key values

   207 >>> p2.article_set = [a4.id, a5.id]

   208 >>> p2.article_set.all()

   209 [<Article: NASA finds intelligent life on Earth>, <Article: Oxygen-free diet works wonders>]

   210 >>> a4.publications.all()

   211 [<Publication: Science News>]

   212 >>> a4.publications = [p3.id]

   213 >>> p2.article_set.all()

   214 [<Article: Oxygen-free diet works wonders>]

   215 >>> a4.publications.all()

   216 [<Publication: Science Weekly>]

   217 

   218 # Recreate the article and Publication we have deleted.

   219 >>> p1 = Publication(id=None, title='The Python Journal')

   220 >>> p1.save()

   221 >>> a2 = Article(id=None, headline='NASA uses Python')

   222 >>> a2.save()

   223 >>> a2.publications.add(p1, p2, p3)

   224 

   225 # Bulk delete some Publications - references to deleted publications should go

   226 >>> Publication.objects.filter(title__startswith='Science').delete()

   227 >>> Publication.objects.all()

   228 [<Publication: Highlights for Children>, <Publication: The Python Journal>]

   229 >>> Article.objects.all()

   230 [<Article: Django lets you build Web apps easily>, <Article: NASA finds intelligent life on Earth>, <Article: NASA uses Python>, <Article: Oxygen-free diet works wonders>]

   231 >>> a2.publications.all()

   232 [<Publication: The Python Journal>]

   233 

   234 # Bulk delete some articles - references to deleted objects should go

   235 >>> q = Article.objects.filter(headline__startswith='Django')

   236 >>> print q

   237 [<Article: Django lets you build Web apps easily>]

   238 >>> q.delete()

   239 

   240 # After the delete, the QuerySet cache needs to be cleared, and the referenced objects should be gone

   241 >>> print q

   242 []

   243 >>> p1.article_set.all()

   244 [<Article: NASA uses Python>]

   245 

   246 # An alternate to calling clear() is to assign the empty set

   247 >>> p1.article_set = []

   248 >>> p1.article_set.all()

   249 []

   250 

   251 >>> a2.publications = [p1, new_publication]

   252 >>> a2.publications.all()

   253 [<Publication: Highlights for Children>, <Publication: The Python Journal>]

   254 >>> a2.publications = []

   255 >>> a2.publications.all()

   256 []

   257 

   258 """}