Gorm: Handling many-to-many relations with *gorm.DB.Create

[RickvanLoo]

I'm currently trying two insert two 'pictures' in table Picture. These pictures have tags, and so share a many-to-many relation with table Tag.

See here the structs:

//Picture is a default DB structure for GORM
type Picture struct {
    ID        int
    Title     string
    URL       string
    CreatedAt time.Time
    UpdatedAt time.Time

    Tags []Tag `gorm:"many2many:picture_tags;"`
}

//Tag is a default DB structure for GORM
type Tag struct {
    ID        int
    Name      string
    CreatedAt time.Time
    UpdatedAt time.Time

    Pictures []Picture `gorm:"many2many:picture_tags;"`
}

With the function MockData i'm putting test data in the Database:

//MockData creates two instants of pictures with 4 tags
func MockData(db *gorm.DB) {
    baby := Tag{
        Name: "Baby",
    }

    cat := Tag{
        Name: "Cat",
    }

    gif := Tag{
        Name: "Gif",
    }

    shit := Tag{
        Name: "Shit",
    }

    picture := Picture{
        Title: "Dancing Baby Gif",
        URL:   "https://45.media.tumblr.com/07106512835c07d2a237a4d12479c7f8/tumblr_mpg5arui7E1s1clzao1_250.gif",
        Tags:  []Tag{baby, gif, shit},
    }

    picture2 := Picture{
        Title: "Dancing Cat Gif",
        URL:   "http://rs143.pbsrc.com/albums/r146/sconti1369/Funny_Pictures_Animated_Dancing_Cat.gif~c200",
        Tags:  []Tag{cat, gif, shit},
    }

    db.Create(&picture)
    db.Create(&picture2)
}

The result should be two pictures in table Picture, and four tags in table Tag, as two tags are shared between the pictures and 2 are unique.

This is not the case. Table Picture is OK:

[
   {
      "ID":1,
      "Title":"Dancing Baby Gif",
      "URL":"https://45.media.tumblr.com/07106512835c07d2a237a4d12479c7f8/tumblr_mpg5arui7E1s1clzao1_250.gif",
      "CreatedAt":"2015-12-08T19:58:55.631519+01:00",
      "UpdatedAt":"2015-12-08T19:58:55.631519+01:00",
      "Tags":null
   },
   {
      "ID":2,
      "Title":"Dancing Cat Gif",
      "URL":"http://rs143.pbsrc.com/albums/r146/sconti1369/Funny_Pictures_Animated_Dancing_Cat.gif~c200",
      "CreatedAt":"2015-12-08T19:58:55.639515+01:00",
      "UpdatedAt":"2015-12-08T19:58:55.639515+01:00",
      "Tags":null
   }
]

(Don't look at the "Tags":null part. I'm not doing a query for them, but it works when requesting only a single ID)

The Tags table is problematic:

[
   {
      "ID":1,
      "Name":"Baby",
      "CreatedAt":"2015-12-08T19:58:55.633513+01:00",
      "UpdatedAt":"2015-12-08T19:58:55.633513+01:00",
      "Pictures":null
   },
   {
      "ID":2,
      "Name":"Gif",
      "CreatedAt":"2015-12-08T19:58:55.636512+01:00",
      "UpdatedAt":"2015-12-08T19:58:55.636512+01:00",
      "Pictures":null
   },
   {
      "ID":3,
      "Name":"Shit",
      "CreatedAt":"2015-12-08T19:58:55.638514+01:00",
      "UpdatedAt":"2015-12-08T19:58:55.638514+01:00",
      "Pictures":null
   },
   {
      "ID":4,
      "Name":"Cat",
      "CreatedAt":"2015-12-08T19:58:55.640522+01:00",
      "UpdatedAt":"2015-12-08T19:58:55.640522+01:00",
      "Pictures":null
   },
   {
      "ID":5,
      "Name":"Gif",
      "CreatedAt":"2015-12-08T19:58:55.642518+01:00",
      "UpdatedAt":"2015-12-08T19:58:55.642518+01:00",
      "Pictures":null
   },
   {
      "ID":6,
      "Name":"Shit",
      "CreatedAt":"2015-12-08T19:58:55.643519+01:00",
      "UpdatedAt":"2015-12-08T19:58:55.643519+01:00",
      "Pictures":null
   }
]

Tag 'Gif' and 'Shit' are being duplicated which they should not be. Searching for tag Gif should return 1 row, and when requesting for this ID should show two linked pictures instead of one.

Removing the Pictures []Picturegorm:"many2many:picture_tags;"`` line from Tag doesn't solve the problem, but makes it more of a hassle to search corresponding pictures from tags. The tags are still being duplicated.

Hope someone can help!

Rivalo

[RickvanLoo]

Fixed this by just creating the Pictures, then the Tags, and then using the .append method (as given in the Many2Many examples) to append the Tag to the Picture.

[danhardman]

I've just run into this issue. I feel db.Create should have a way of using existing records in relationships such as this, provided there is a unique/unique_index field specified on (in this example) Tag.