go-pg - pg: can't find dst value for model id=","

getting pg: can't find dst value for model id=","

I have defined the following models

// omitting fields which don't seem relevant to the issue
// corresponding queries also shortened as appropriate
type GrProduct struct {
	tableName        struct{} `sql:"gr_product"`
	ID               int64
	Name             string
// fk:Product,joinFK:Category given so that joins are made on category_id and product_id with gr_product_category_mapping
	Categories       []*GrCategory               `pg:",many2many:gr_product_category_mapping,fk:Product,joinFK:Category"`
	CategoryMappings []*GrProductCategoryMapping `pg:",fk:Product"`
}

type GrProductCategoryMapping struct {
	tableName  struct{} `sql:"gr_product_category_mapping"`
	ID         int64
	ProductID  int64 // product_id in db instead of gr_product_id
	CategoryID int // category id in db instead of gr_category_id
	IsPrimary  bool
}

type GrCategory struct {
	tableName                struct{} `sql:"gr_category"`
	ID                       int
	Name                     string
	Products                 []*GrProduct `pg:",many2many:gr_product_category_mapping,fk:Category,joinFK:Product"`
}

on trying this -

p := models.GrProduct{}
if err := models.DB.Model(&p).
	Column("Categories").
	Where("id = ?", 10).
	Select(); err != nil {
	panic(err)
}

These are the queries that are made

SELECT
    "gr_product"."id",
    "gr_product"."name"  
FROM
    gr_product AS "gr_product" 
WHERE
    (
        id= 10
    );

SELECT
    gr_product_category_mapping.*,
    "gr_category"."id",
    "gr_category"."name"
    
FROM
    gr_category AS "gr_category" 
JOIN
    gr_product_category_mapping AS gr_product_category_mapping 
        ON (
            gr_product_category_mapping."product_id"
        ) IN (
            (
                10
            )
        ) 
WHERE
    (
        "gr_category"."id" = gr_product_category_mapping."category_id"
    );

I get panic: pg: can't find dst value for model id="," on line https://github.com/go-pg/pg/blob/master/orm/model_table_m2m.go#L53 I think.
On trying to dig deeper with delve I found that that the 'prefix' m.baseTable.ModelName+"_" evaluates to gr_product_, but instead probably should be product_, since columns contains

map[string]string [
        "product_id": "10",
        "category_id": "48",
        "is_primary": "t",
]

I haven't been able to figure out how to override this default behaviour(new to both Golang and go-pg), any help would be appreciated, thanks

You should be able to use the sql tag to override the default column names.

type GrProductCategoryMapping struct {
    tableName  struct{} `sql:"gr_product_category_mapping"`
    ID         int64
    ProductID  int64 `sql:"product_id"`
    CategoryID int   `sql:"category_id"`
    IsPrimary  bool
}

Read more here.

this was a bug in which is fixed in v6.4.6.
here's the relevant issue - https://github.com/go-pg/pg/issues/583