After I saw this discussion, I managed to write a generic function for this

const result = await this.db.select({ 
      ...getTableColumns(this.firstSchema), 
      [this.firstFieldName]: sql'json_agg(${this.secondSchema})' 
   }) 
   .from(this.firstSchema) 
   .leftJoin(this.secondSchema, eq(this.firstSchema[this.firstFieldId], this.secondSchema.id))
   .groupBy(this.firstSchema.id)