go-pg how to select also entity from related table? - go

type Book struct {
tableName struct{} `pg:"book" json:"-"`
Id int `pg:"id,pk" json:"id"`
Author int `pg:"author_id,notnull" json:"-"`
Author *Author `pg:"fk:author_id" json:"author,omitempty"`
}
I want select book and author in one query.
If I try this:
var r []model.Book
_, err := dao.FusedDb.Query(&r, `SELECT * FROM book b INNER JOIN author a on a.id = b.author_id`)
I get an error
pg: can't find column=name in model=Book (try discard_unknown_columns)

I wrote down a piece of code that I always use when I've to deal with this scenario. First, let me show the code and then I'll comment on the relevant parts:
package main
import (
"database/sql"
"fmt"
_ "github.com/lib/pq"
)
type Book struct {
gorm.Model
Title string
Description string
AuthorID uint
Author Author
}
type Author struct {
gorm.Model
FirstName string
LastName string
Books []Book
}
type Result struct {
BookId int
AuthorId int
Title string
FirstName string
LastName string
}
func main() {
conn, err := sql.Open("postgres", "host=localhost user=postgres password=postgres dbname=postgres port=5432 sslmode=disable")
if err != nil {
panic(err)
}
defer conn.Close()
// query
var result []Result
rows, err := conn.Query("select b.id, a.id, b.title, a.first_name, a.last_name from authors a inner join books b on a.id = b.author_id")
if err != nil {
panic(err)
}
defer rows.Close()
for rows.Next() {
var record Result
if err := rows.Scan(&record.BookId, &record.AuthorId, &record.Title, &record.FirstName, &record.LastName); err != nil {
panic(err)
}
result = append(result, record)
}
if err := rows.Err(); err != nil {
panic(err)
}
fmt.Printf("%v", result)
}
Structs definition
The Book and Author structs represent the tables defined in my database. Result is used to hold the fetched records through the query specified below.
The query
The query is pretty straightforward. We only used the method Query on the SQL client opened at the beginning of the main function. Then, we've to defer a call to the method Close on the rows variable to clean up.
Scanning
The for makes sure that we scan all of the rows retrieved with the Query method. To understand if there are other rows to fetch we use the method Next that returns a bool value indicating whether or not there are other rows to scan.
In the body of the for we declare a loop-scoped variable to hold the current record. Thanks to the Scan method we'll be able to assign each column to the relative field of the struct.
Lastly, we've to check for any error by invoking the method Err on the rows variable and handle it.
Let me know if this clarifies your question, thanks!

Related

Get data from two different struct

Have this struct of User and Post and I try to make Name from User to be included within Post Struct when a user create a new post.
type User struct {
ID int
Name string
Created time.Time
}
type Post struct {
ID int
PostTitle string
PostDesc string
Created time.Time
}
How can I create something connected between this two struct such as Author of the Post ?
The goal is try to get the name of the post author which from User struct with the code below:
post, err := app.Models.Posts.GetPost(id)
GetPost() just run SELECT query and scan row
This approach is without any ORM.
It's a simple query that can return multiple rows. You've to scan the whole resultset and map each column on the struct's fields.
Keep in mind to always check for errors.
Below you can find the solution:
package main
import (
"database/sql"
"fmt"
"time"
_ "github.com/lib/pq"
)
type Post struct {
ID int
PostTitle string
PostDesc string
Created time.Time
UserID int
User User
}
type User struct {
ID int
Name string
Created time.Time
}
func main() {
conn, err := sql.Open("postgres", "host=localhost user=postgres password=postgres dbname=postgres port=5432 sslmode=disable")
if err != nil {
panic(err)
}
defer conn.Close()
// get MAX id
var id int
conn.QueryRow(`SELECT MAX(id) FROM posts`).Scan(&id)
// insert
sqlInsertStmt := `INSERT INTO posts (id, post_title, post_desc, created, user_id) VALUES ($1,$2,$3,$4,$5)`
if _, err = conn.Exec(sqlInsertStmt, id+1, "TDD", "Introduction to Test-Driven-Development", time.Now(), 1); err != nil {
panic(err)
}
// read
rows, err := conn.Query(`SELECT posts.id, post_title, post_desc, posts.created, users.id, users.name, users.created FROM posts INNER JOIN users ON posts.user_id=users.id`)
if err != nil {
panic(err)
}
var posts []Post
for rows.Next() {
var post Post
if err = rows.Scan(&post.ID, &post.PostTitle, &post.PostDesc, &post.Created, &post.User.ID, &post.User.Name, &post.User.Created); err != nil {
panic(err)
}
posts = append(posts, post)
}
if err = rows.Err(); err != nil {
panic(err)
}
for _, v := range posts {
fmt.Printf("author name: %q\n", v.User.Name)
}
}
Let me know if this helps.
Edit
I've also included an example of INSERT in Postgres. To achieve it, we've to use the db.Exec() function, and provide the parameters.
Pay attention to how you construct the query as you can get a SQL-Injection vulnerability.
Lastly, in a real-world scenario, you shouldn't lookup for the MAX id in the posts table but should be automatically generated.
Give it a try to this solution, maybe it resolves your issue.
First, I defined the structs in this way:
// "Post" belongs to "User", "UserID" is the foreign key
type Post struct {
gorm.Model
ID int
PostTitle string
PostDesc string
Created time.Time
UserID int
User User
}
type User struct {
ID int
Name string
Created time.Time
}
In this way, you can say that Post belongs to User and access the User's information within the Post struct.
To query the records, you've to use Preload("User") to be sure to eager load the User records from the separate table.
Keep attention to the name you pass in as the argument in Preload, as it can be tricky.
Lastly, you'll be able to access data in the embedded struct (with the dot notation).
Below, you can find a complete working example (implemented with the use of Docker):
package main
import (
"fmt"
"time"
"gorm.io/driver/postgres"
"gorm.io/gorm"
)
// "Post" belongs to "User", "UserID" is the foreign key
type Post struct {
gorm.Model
ID int
PostTitle string
PostDesc string
Created time.Time
UserID int
User User
}
type User struct {
ID int
Name string
Created time.Time
}
func main() {
dsn := "host=localhost user=postgres password=postgres dbname=postgres port=5432 sslmode=disable"
db, err := gorm.Open(postgres.Open(dsn), &gorm.Config{})
if err != nil {
panic(err)
}
db.AutoMigrate(&Post{})
newPost := &Post{ID: 1, PostTitle: "Golang", PostDesc: "Introduction to Golang", Created: time.Now(), UserID: 1, User: User{ID: 1, Name: "John Doe", Created: time.Now()}}
db.Create(newPost)
var post Post
db.Preload("User").Find(&post, 1)
fmt.Printf("author name: %q\n", post.User.Name)
}
Let me know if I answered your question!

How to get and store the document ID using a struct

I've defined a data structure like so:
type Person struct {
Name string `firestore:"name,omitempty"`
}
When I query all the documents in a collection I'd like to be able to attach the ID to the documents for later reference, but not necessarily have ID as an attribute stored in Firestore (unless its the only way). In javascript or python this is straightforward as the data structures are dynamic and I can just query the ID post get() and add it as a dynamic key/value. myObj.id = doc.id
How would I do this with Go?
package main
import (
"fmt"
"cloud.google.com/go/firestore"
"context"
"google.golang.org/api/iterator"
"log"
)
type Person struct {
Name string `firestore:"name,omitempty"`
}
func main() {
ctx := context.Background()
c, err := firestore.NewClient(ctx, "my-project")
if err != nil {
log.Fatalf("error: %v", err)
}
var people []Person
iter := c.Collection("people").Documents(ctx)
for {
doc, err := iter.Next()
if err == iterator.Done {
break
}
if err != nil {
log.Fatalf("error: %v", err)
}
var p Person
err = doc.DataTo(p)
if err != nil {
log.Fatalf("error: %v", err)
}
// id := doc.Ref.ID
people = append(people, p)
}
fmt.Println(people)
}
output, no ID
>> [{John Smith}]
I believe that the firestore struct tags works the same way as the tags in the encoding/json package. So a tag with a value of "-" would mean ignore the field.
So
type Person struct {
ID int `firestore:"-"`
Name string `firestore:"name,omitempty"`
}
should do the trick.
You can set ID yourself, but the firestore pkg will ignore it when reading/writing data.
If you want to store the firestore document ID on the Person type, your struct must have a declared field for it.
Golang firestore docs don't mention this explicitly, but since a firestore doc ID is not part of the document fields, the func (*DocumentSnapshot) DataTo does not populate the ID. Instead, you may get the document ID from the DocumentRef type and add it to Person yourself.
The doc also states that:
Note that this client supports struct tags beginning with "firestore:" that work like the tags of the encoding/json package, letting you rename fields, ignore them, or omit their values when empty
Therefore, if you want to omit the ID when marshaling back for firestore, your could use the tag firestore:"-"
The Person would look like this:
type Person struct {
ID string `firestore:"-"`
Name string `firestore:"name,omitempty"`
}
inside the loop:
var p Person
err := docSnap.DataTo(&p)
if err != nil {
// handle it
}
p.ID = doc.Ref.ID

How can I do conditional actions in SQLX

I have database store function:
func (p *ProductsRep) FindAll(PageNumber int, PaginationSize int, Query string) []*postgresmodels.Product {
Also I have SQL query look like this:
SELECT * FROM table_name.
Then I want to concat conditional action like WHERE some_value=3 if some value (in this case Query) exists then I want to get SELECT * FROM table_name WHERE some_value=3.
I tried to use fmt.Sprintf to concat, or strings.Join, or bytes.Buffer.WriteString. But everytime I getting this error:
I replace real value for understanding:
pq: column "Some value" does not exist.
How can I do "adaptive" queries, which depends on inputed function values.
I believe you are trying to query rows in the database by using parameters.
You need to make sure you don't pass this data in as RAW values, due to the potential risk of SQL injection. You can make queries by using store procedures
You can use the function Query to pass in your query with your parameters. In the example case this is $1. If you wanted to you could add $2, $3... etc depending on how many parameters you wanted to query
Here is two examples
Postgres
using "github.com/jackc/pgx/v4" driver
ctx := context.Background()
type Bar struct {
ID int64
SomeValue string
}
rows, err := conn.Query(ctx, `SELECT * FROM main WHERE some_value=$1`, "foo")
if err != nil {
fmt.Println("ERRO")
panic(err) // handle error
}
defer rows.Close()
var items []Bar
for rows.Next() {
var someValue string
var id int64
if err := rows.Scan(&id, &someValue); err != nil {
log.Fatal(err) // handle error
}
item := Bar{
ID: id,
SomeValue: someValue,
}
items = append(items, item)
}
fmt.Println(items)
MySQL Driver
https://golang.org/pkg/database/sql/#DB.QueryRow
type Bar struct {
ID int64
SomeValue string
}
rows, err := conn.Query(`SELECT * FROM main WHERE some_value=$1`, "foo")
if err != nil {
fmt.Println("ERRO")
panic(err) // handle error
}
defer rows.Close()
var items []Bar
for rows.Next() {
var someValue string
var id int64
if err := rows.Scan(&id, &someValue); err != nil {
log.Fatal(err) // handle error
}
item := Bar{
ID: id,
SomeValue: someValue,
}
items = append(items, item)
}
fmt.Println(items)

Create json fro Cassandra in go

I have a table in Cassandra defined as the following :
CREATE TABLE book.book
(
title text PRIMARY KEY,
amount decimal,
available int,
createdon timestamp
)
I am trying to select * from that table and return the values in json format. I am able to achieve that using
type Book struct {
Title string `json:"title"`
Amount inf.Dec `json:"amount"`
CreatedOn time.Time `json:"createdon"`
Available int `json:"available"`
}
with
func cassandraDisplay(query string, w http.ResponseWriter) {
cluster := gocql.NewCluster("xxxxxxxx:xxxx")
session, _ := cluster.CreateSession()
defer session.Close()
iter := session.Query("SELECT * FROM book.book").Iter()
var book Book
for iter.Scan(&book.Title ,&book.Amount ,&book.CreatedOn,&book.Available{
fmt.Println(book.Title , book.Amount,book.CreatedO,book.Available)
j, ERR:= json.Marshal(&iter)
if ERR != nil {panic(ERR)}
//do things with j
}
if err := iter.Close(); err != nil {log.Fatal(err)}
}
but the requirement require a dynamic and no hard coding any info; since it is http service and the query will be passed through the url.
Any idea how to get this to work?
#Michael,
You may want to use MapScan: https://godoc.org/github.com/gocql/gocql#Iter.MapScan
This is as abstract as it can get.
From https://github.com/gocql/gocql/blob/master/cassandra_test.go:
...
testMap := make(map[string]interface{})
if !session.Query(`SELECT * FROM slice_map_table`).Iter().MapScan(testMap) {
t.Fatal("MapScan failed to work with one row")
}
...
And after that you'll need to reflect/explore map content, but that's a different topic.

Mapping struct to mysql table, and binding row to struct

This is my first script using go-sql-driver.
My mysql table (PRODUCT) looks like:
id int
name varchar(255)
IsMatch tinyint(1)
created datetime
I want to simply load a row from a table, and bind it to a struct.
I have this so far:
package main
import (
"database/sql"
"fmt"
_ "github.com/go-sql-driver/mysql"
)
type Product struct {
Id int64
Name string
IsMatch ??????????
Created ?????
}
func main() {
fmt.Printf("hello, world!\n")
db, err := sql.Open("mysql", "root:#/product_development")
defer db.Close()
err = db.Ping()
if err != nil {
panic(err.Error()) // proper error handling instead of panic in your app
}
rows, err := db.Query("SELECT * FROM products where id=1")
if err != nil {
panic(err.Error()) // proper error handling instead of panic in your app
}
}
Now I need to:
1. What datatype in Go do I use for tinyint and datetime?
2. How to I map the rows to a Product struct?
What datatype in Go do I use for tinyint and datetime?
For a hint as to the types that the database/sql package will be using, have a look at the documentation for database/sql.Scanner, which lists the Go types used within database/sql itself:
int64
float64
bool
[]byte
string
time.Time
nil - for NULL values
This would lead you to try int64 for IsMatch and time.Time for Created. I believe in reality you can use pretty much any sized int (maybe even bool, you'd have to check the source) for IsMatch because it can be stored "without loss of precision." The documentation for go-mysql-driver explains that you will need to add parseTime=true to your DSN in order for it to parse into a time.Time automatically or use NullTime.
How to I map the rows to a Product struct?
It should be something pretty strightforward, using Rows.Scan, like:
var products []*Product
for rows.Next() {
p := new(Product)
if err := rows.Scan(&p.ID, &p.Name, &p.IsMatch, &p.Created); err != nil { ... }
products = append(products, p)
}
if err := rows.Err() { ... }
This scans the columns into the fields of a struct and accumulates them into a slice. (Don't forget to Close the rows!)
How to I map the rows to a Product struct?
You can use reflect to bind table rows in db to a struct, and
automatically match values without long Hard-Code sql string which is easy to make mistakes.
this is a light demo: sqlmapper

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