Cast Join Value

[comsma]

I would like to define a has-one relation between InvTran and OeHdr and i was able to define the join successfully. Although it gives me the data i need it takes nearly 300ms to run. The reason why the data types are not both varchar is that i am building bun on top of a database for a ERP system i do not have control over and i need to follow the existing tables.

I took the generated query

LEFT JOIN "oe_hdr" AS "oe_hdr" ON ("oe_hdr"."order_no" = "inv_tran"."document_no")

and was able to use a CAST method as follows...
LEFT JOIN "oe_hdr" AS "oe_hdr" ON ("oe_hdr"."order_no" = CAST("inv_tran"."document_no" as varchar)).

This reduces the query time down to 20ms; Im not entirely sure how i can CAST the document_no using tags in bun or writing a custom relation function.

Here is my models and a example function

package model
type InvTran struct {
	bun.BaseModel     `bun:"table:inv_tran"`
	TransactionNumber float64 `bun:"transaction_number,type:decimal(19),pk"`
	InvMastUid        int32   `bun:"inv_mast_uid,type:int"`
	QtyAllocated      float64 `bun:"qty_allocated,type:decimal(19,9)"`
	UnitOfMeasure     string  `bun:"unit_of_measure,type:varchar(8)"`
	DocumentNo        float64 `bun:"document_no,type:decimal(19)"`
	SubDocumentNo     float64 `bun:"sub_document_no,type:decimal(19)"`
	TransType         string  `bun:"trans_type,type:varchar(5)"`

	OeHdr *OeHdr `bun:"rel:has-one,join:document_no=order_no"`
}

type OeHdr struct {
	bun.BaseModel        `bun:"table:oe_hdr"`
	OrderNo              string         `bun:"order_no,type:varchar(8),pk"`
	Ship2Name            sql.NullString `bun:"ship2_name,type:varchar(50)"`
	Ship2Add1            sql.NullString `bun:"ship2_add1,type:varchar(50)"`
	Ship2Add2            sql.NullString `bun:"ship2_add2,type:varchar(50)"`
	Ship2City            sql.NullString `bun:"ship2_city,type:varchar(50)"`
	Ship2State           sql.NullString `bun:"ship2_state,type:varchar(50)"`
	Ship2Zip             sql.NullString `bun:"ship2_zip,type:varchar(10)"`
	Ship2Country         sql.NullString `bun:"ship2_country,type:varchar(50)"`
	ContactId            sql.NullString `bun:"contact_id,type:varchar(16)"`
	DeliveryInstructions sql.NullString `bun:"delivery_instructions,type:varchar(255)"`
	PoNo                 sql.NullString `bun:"po_no,type:varchar(50)"`

	OeLineItems []OeLine `bun:"rel:has-many,join:order_no=order_no"`
	Contact     Contact  `bun:"rel:has-one,join:contact_id=id"`
}

package repository
invTran :=new(model.InvTran)
err := r.bun.NewSelect().Model(invTran).
		Relation("OeHdr").
		Where(
			"sub_document_no = ?",
			1,
		).Scan(ctx)
	if err != nil {
		fmt.Print(err)
		return nil, err
	}