Inserting and selecting PostGIS Geometry with Gorm

Question

I\'ve been trying to find a way to insert and retrieve geometric types using Golang, and specifically the library gorm. I\'m also attempting to use the library orb that defines

Answer 1

The solution I ended up using was as follows:

First I created new types that wrapped all of the orb types, for example:

type Polygon4326 orb.Polygon
type Point4326 orb.Point

Then I implemented the Scan(), Value() methods on each type. I had to however edit the bytes and convert to/from hexadecimal. When you directly query on a spatial column in PostGIS, it will return a hexadecimal representation of EWKB, essentially WKB, but including 4 bytes to represent the projection ID (in my case 4326).

Before inserting, I had to add the bytes that represent the projection of 4326.

Before reading, I had to strip those bytes, since orb's built in scanning expected WKB format.

Answer 2

I used @robbieperry22's answer with a different encoding library and found I didn't need to tinker with bytes at all.

Included gist for reference.

import  "github.com/twpayne/go-geom/encoding/geojson"


type EWKBGeomPoint geom.Point

func (g *EWKBGeomPoint) Scan(input interface{}) error {
    gt, err := ewkb.Unmarshal(input.([]byte))
    if err != nil {
        return err
    }
    g = gt.(*EWKBGeomPoint)

    return nil
}

func (g EWKBGeomPoint) Value() (driver.Value, error) {
    b := geom.Point(g)
    bp := &b
    ewkbPt := ewkb.Point{Point: bp.SetSRID(4326)}
    return ewkbPt.Value()
}


type Track struct {
    gorm.Model

    GeometryPoint EWKBGeomPoint `gorm:"column:geom"`
}

And then used a little customization on the table set up/migrate part:

err = db.Exec(`CREATE TABLE IF NOT EXISTS tracks (
    id SERIAL PRIMARY KEY,
    geom geometry(POINT, 4326) NOT NULL
);`).Error
if err != nil {
    return err
}

err = gormigrate.New(db, gormigrate.DefaultOptions, []*gormigrate.Migration{
{
    ID: "init",
    Migrate: func(tx *gorm.DB) error {
        return tx.CreateTable(
            Tables...,
        ).Error
    },
},
{
    ID: "tracks_except_geom",
    Migrate: func(tx *gorm.DB) error {
        return db.AutoMigrate(Track{}).Error
    },
},
}).Migrate()

Answer 3

Is it possible to make some sort of trigger or rule that would automatically call the needed functions on the data coming in/out?

Ever tried gorm hooks, example:

type Example struct {
    ID   int
    Name string
    Geom ...
}

func (e *Example) AfterFind() (err error) {
    e.Geom = ... // Do whatever you like here
    return
}

There is a handful of hooks that you can use. I find them pretty neat and useful.

Answer 4

Another solution, which I ended up using was with go-geos, as I discovered I needed to use the GEOS C library. With that, I am able to convert the struct into WKT for inserting (as postgis accepts it as regular text) and convert from WKB when scanning.

type Geometry4326 *geos.Geometry

// Value converts the given Geometry4326 struct into WKT such that it can be stored in a 
// database. Implements Valuer interface for use with database operations.
func (g Geometry4326) Value() (driver.Value, error) {

    str, err := g.ToWKT()
    if err != nil {
        return nil, err
    }

    return "SRID=4326;" + str, nil
}

// Scan converts the hexadecimal representation of geometry into the given Geometry4326 
// struct. Implements Scanner interface for use with database operations.
func (g *Geometry4326) Scan(value interface{}) error {

    bytes, ok := value.([]byte)
    if !ok {
        return errors.New("cannot convert database value to geometry")
    }

    str := string(bytes)

    geom, err := geos.FromHex(str)
    if err != nil {
        return errors.Wrap(err, "cannot get geometry from hex")
    }

    geometry := Geometry4326(geom)
    *g = geometry

    return nil
}

This solution might not be ideal for everyone as not everyone needs to use the GEOS C library, which can be a pain to get working on windows. I'm sure though that the same thing can be accomplished using different libraries.

I additionally implemented UnmarshalJSON() and MarshalJSON() on the struct so that it can automatically Marshal/Unmarshal GeoJSON, and then save/get from the database seamlessly. I accomplished this using geojson-go to convert GeoJSON to/from a struct, and then geojson-geos-go to convert said struct into the go-geos struct I was using. A little convoluted, yes, but it works.