Suppose I have the following code:
type User struct {
ID string
Username string
Name string
Password string
}
What I want to do is create another struct that can access certain fields from the User struct, instead of accessing all of it, to prevent people from seeing the password, for example.
This does not work:
type Note struct {
ID string
Text string
UserID User.ID
}
Is there any way to do this, or do I simply create the Note.UserID field to have the same data type as the ID in the User struct?
Assuming the types are in different packages you can do this by exporting vs not exporting the fields. A field who's name begins with a lower case letter is not exported, meaning it is not visible outside the package where it is declared/defined. So, in this case if the user existed in one package, call it user while the other type were declared in another you could accomplish this 'hiding' of properties by changing the definition to;
type User struct {
ID string
username string
name string
password string
}
If the two types live in the same package there is no way of making a field private/hidden/ect, everything will be available in that scope.
The Go Programming Language Specification
Exported identifiers
An identifier may be exported to permit access to it from another
package. An identifier is exported if both:
the first character of the identifier's name is a Unicode upper case
letter (Unicode class "Lu"); and
the identifier is declared in the package block or it is a field name
or method name.
All other identifiers are not exported.
Give User its own package and don't export the password.
For example,
package user
type User struct {
ID string
Username string
Name string
password string
}
func (u *User) IsPassword(password string) bool {
return password == u.password
}
Related
Why we don't call person field as embedded?
“type user struct {
name string
email string
}
type admin struct {
person user // NOT Embedding
level string
}”
But in other cases like below we call it embedded:
“type user struct {
name string
email string
}
type admin struct {
user // Value Semantic Embedding
level string
}”
What I think is that person is also embedded like value/pointer semantic embedding. What I'm missing here?
Because that's how the Go language specification defines it:
A field declared with a type but no explicit field name is called an embedded field.
I can see how the term "embedded" would be confusing. After all, named and unnamed fields end up with the same memory layout, "embedded" into the parent struct. "Anonymous field" might have been a better name for it, but that's not the name that the Go language designers chose.
With the first code you can't treat an admin object as a user object, like using member access or type assertion. This also affects how an embedding struct satisfies interfaces.
For example, the following code works with proper embedding, but not a simple member struct:
var a admin
a.name = "asdfg"
u := a.(user)
Why can I do this
package main
import "fmt"
func main() {
c := Circle{x: 0, y: 0, r: 5}
fmt.Println(c.r)
}
type Circle struct {
x float64
y float64
r float64
}
http://play.golang.org/p/0ypcekVDV9
When I can't do the same with a struct in a package?
If I would try to access a struct with a field with lowercase a compiler error is returned..
As already stated, the fields need to be exported to be accessible from another package. See the spec
Exported identifiers
An identifier may be exported to permit access to it from another
package. An identifier is exported if both:
the first character of the identifier's name is a Unicode upper case
letter (Unicode class "Lu");
and the identifier is declared in the
package block or it is a field name or method name. All other
identifiers are not exported.
If you want to keep the fields private, you need to use accessor (set/get) methods which you can read about here
Getters
Go doesn't provide automatic support for getters and setters. There's
nothing wrong with providing getters and setters yourself, and it's
often appropriate to do so, but it's neither idiomatic nor necessary
to put Get into the getter's name. If you have a field called owner
(lower case, unexported), the getter method should be called Owner
(upper case, exported), not GetOwner. The use of upper-case names for
export provides the hook to discriminate the field from the method. A
setter function, if needed, will likely be called SetOwner. Both names
read well in practice:
owner := obj.Owner()
if owner != user {
obj.SetOwner(user)
}
If the struct is in a different package than the main function, then you cannot access the struct's private fields from that main function.
That is what "private" means.
I have two structs that represent models that will be inserted into a mongodb database. One struct (Investment) has the other struct (Group) as one of its fields.
type Group struct {
Base
Name string `json:"name" bson"name"`
}
type Investment struct {
Base
Symbol string `json:"symbol" bson:"symbol" binding:"required"`
Group Group `json:"group" bson:"group"`
Fields bson.M `json:"fields" bson:"fields"`
}
The problem I'm having is that in the Investment model, Group is not required. If there is no group, I think its better for it to not be inserted in the db. Whats the best way to handle a db model such as this in Go?
tl;dr: Use ,omitempty, and if you need to worry about the difference between a zero value and null/not specified, do what the GitHub API does and use a pointer.
Both json and bson support the ,omitempty tag. For json, "empty values are false, 0, any nil pointer or interface value, and any array, slice, map, or string of length zero" (json docs). For bson, ,omitempty means "Only include the field if it's not set to the zero value for the type or to empty slices or maps", and zero values include empty strings and nil pointers (bson docs).
So if you really need a Group struct, you can put a *Group in instead, and it won't be stored when the pointer is nil. If Investment only needs to hold the group's name, it's even simpler: "" as group name keeps a group key from being stored.
bson defaults to using the lowercased field name already so you can omit that from the struct tag when they match. json will default to the Capitalized name, so specify the lowercase name in a tag if you need lowercase.
So, best case, maybe you can just use:
type Investment struct {
Base
Symbol string `json:"symbol" binding:"required"`
Group string `json:"group,omitempty" bson:",omitempty"`
Fields bson.M `json:"fields"`
}
If you ever run into fields where the zero value for the type ("", 0, false, etc.) is distinct from "not specified", you can do what the GitHub API does and put pointers in your structures--essentially an extension of the *Group trick.
Avoid strut fields to marshal if they are empty -
A struct field may be primitive type(string, int, bool etc) or even an another struct type.
So sometimes we don't want a struct's field to
go in json data(may to database insertion or in external api call) if they are empty
Example:
type Investment struct {
Base
Symbol string `json:"symbol" bson:"symbol" binding:"required"`
Group Group `json:"group" bson:"group"`
Fields bson.M `json:"fields" bson:"fields"`
}
If we want that Symbol and Group might contain empty values(0, false, nil pointer, zero size interface/struct) then we can avoid them in json marshaling like below.
type Investment struct {
Base
Symbol string `json:"symbol,omitempty" bson:"symbol,omitempty" binding:"required"`
Group *Group `json:"group,omitempty" bson:"group,omitempty"`
Fields bson.M `json:"fields" bson:"fields"`
}
Her "Group" field is pointer to Group struct and whenever it will point to nil pointer it will be omitted from json marshaling.
And obviously we would be filling values in Group field like below.
// declared investment variable of type Investment struct
investment.Group = &groupData
I am trying to print json result from struct I created as following:
type Machine struct {
m_ip string
m_type string
m_serial string
}
and print out
m:= &Machine{ m_ip:"test", m_type:"test", m_serial:"test" }
m_json:= json.Marshal(m)
fmt.Println(m_json)
However, result returned just {}
Secondly,I tried to changed the first letter of words to Uppercase as follow:
type Machine struct{
MachIp string
MachType string
MachSerial string
}
and it works! Why doesn't the word with lowercase character at the front work, anyway?
Go uses case to determine whether a particular identifier is public or private within the context of your package. In your first example, the fields are not visible to json.Marshal because it is not part of the package containing your code. When you changed the fields to be upper case, they became public so could be exported.
If you need to use lower case identifiers in your JSON output though, you can tag the fields with the desired identifiers. For example:
type Machine struct{
MachIp string `json:"m_ip"`
MachType string `json:"m_type"`
MachSerial string `json:"m_serial"`
}
In the Go Language Specification, it mentions a brief overview of tags:
A field declaration may be followed by an optional string literal tag,
which becomes an attribute for all the fields in the corresponding
field declaration. The tags are made visible through a reflection
interface but are otherwise ignored.
// A struct corresponding to the TimeStamp protocol buffer.
// The tag strings define the protocol buffer field numbers.
struct {
microsec uint64 "field 1"
serverIP6 uint64 "field 2"
process string "field 3"
}
This is a very short explanation IMO, and I was wondering if anyone could provide me with what use these tags would be?
A tag for a field allows you to attach meta-information to the field which can be acquired using reflection. Usually it is used to provide transformation info on how a struct field is encoded to or decoded from another format (or stored/retrieved from a database), but you can use it to store whatever meta-info you want to, either intended for another package or for your own use.
As mentioned in the documentation of reflect.StructTag, by convention the value of a tag string is a space-separated list of key:"value" pairs, for example:
type User struct {
Name string `json:"name" xml:"name"`
}
The key usually denotes the package that the subsequent "value" is for, for example json keys are processed/used by the encoding/json package.
If multiple information is to be passed in the "value", usually it is specified by separating it with a comma (','), e.g.
Name string `json:"name,omitempty" xml:"name"`
Usually a dash value ('-') for the "value" means to exclude the field from the process (e.g. in case of json it means not to marshal or unmarshal that field).
Example of accessing your custom tags using reflection
We can use reflection (reflect package) to access the tag values of struct fields. Basically we need to acquire the Type of our struct, and then we can query fields e.g. with Type.Field(i int) or Type.FieldByName(name string). These methods return a value of StructField which describes / represents a struct field; and StructField.Tag is a value of type StructTag which describes / represents a tag value.
Previously we talked about "convention". This convention means that if you follow it, you may use the StructTag.Get(key string) method which parses the value of a tag and returns you the "value" of the key you specify. The convention is implemented / built into this Get() method. If you don't follow the convention, Get() will not be able to parse key:"value" pairs and find what you're looking for. That's also not a problem, but then you need to implement your own parsing logic.
Also there is StructTag.Lookup() (was added in Go 1.7) which is "like Get() but distinguishes the tag not containing the given key from the tag associating an empty string with the given key".
So let's see a simple example:
type User struct {
Name string `mytag:"MyName"`
Email string `mytag:"MyEmail"`
}
u := User{"Bob", "bob#mycompany.com"}
t := reflect.TypeOf(u)
for _, fieldName := range []string{"Name", "Email"} {
field, found := t.FieldByName(fieldName)
if !found {
continue
}
fmt.Printf("\nField: User.%s\n", fieldName)
fmt.Printf("\tWhole tag value : %q\n", field.Tag)
fmt.Printf("\tValue of 'mytag': %q\n", field.Tag.Get("mytag"))
}
Output (try it on the Go Playground):
Field: User.Name
Whole tag value : "mytag:\"MyName\""
Value of 'mytag': "MyName"
Field: User.Email
Whole tag value : "mytag:\"MyEmail\""
Value of 'mytag': "MyEmail"
GopherCon 2015 had a presentation about struct tags called:
The Many Faces of Struct Tags (slide) (and a video)
Here is a list of commonly used tag keys:
json - used by the encoding/json package, detailed at json.Marshal()
xml - used by the encoding/xml package, detailed at xml.Marshal()
bson - used by gobson, detailed at bson.Marshal(); also by the mongo-go driver, detailed at bson package doc
protobuf - used by github.com/golang/protobuf/proto, detailed in the package doc
yaml - used by the gopkg.in/yaml.v2 package, detailed at yaml.Marshal()
db - used by the github.com/jmoiron/sqlx package; also used by github.com/go-gorp/gorp package
orm - used by the github.com/astaxie/beego/orm package, detailed at Models – Beego ORM
gorm - used by gorm.io/gorm, examples can be found in their docs
valid - used by the github.com/asaskevich/govalidator package, examples can be found in the project page
datastore - used by appengine/datastore (Google App Engine platform, Datastore service), detailed at Properties
schema - used by github.com/gorilla/schema to fill a struct with HTML form values, detailed in the package doc
asn - used by the encoding/asn1 package, detailed at asn1.Marshal() and asn1.Unmarshal()
csv - used by the github.com/gocarina/gocsv package
env - used by the github.com/caarlos0/env package
Here is a really simple example of tags being used with the encoding/json package to control how fields are interpreted during encoding and decoding:
Try live: http://play.golang.org/p/BMeR8p1cKf
package main
import (
"fmt"
"encoding/json"
)
type Person struct {
FirstName string `json:"first_name"`
LastName string `json:"last_name"`
MiddleName string `json:"middle_name,omitempty"`
}
func main() {
json_string := `
{
"first_name": "John",
"last_name": "Smith"
}`
person := new(Person)
json.Unmarshal([]byte(json_string), person)
fmt.Println(person)
new_json, _ := json.Marshal(person)
fmt.Printf("%s\n", new_json)
}
// *Output*
// &{John Smith }
// {"first_name":"John","last_name":"Smith"}
The json package can look at the tags for the field and be told how to map json <=> struct field, and also extra options like whether it should ignore empty fields when serializing back to json.
Basically, any package can use reflection on the fields to look at tag values and act on those values. There is a little more info about them in the reflect package
http://golang.org/pkg/reflect/#StructTag :
By convention, tag strings are a concatenation of optionally
space-separated key:"value" pairs. Each key is a non-empty string
consisting of non-control characters other than space (U+0020 ' '),
quote (U+0022 '"'), and colon (U+003A ':'). Each value is quoted using
U+0022 '"' characters and Go string literal syntax.
It's some sort of specifications that specifies how packages treat with a field that is tagged.
for example:
type User struct {
FirstName string `json:"first_name"`
LastName string `json:"last_name"`
}
json tag informs json package that marshalled output of following user
u := User{
FirstName: "some first name",
LastName: "some last name",
}
would be like this:
{"first_name":"some first name","last_name":"some last name"}
other example is gorm package tags declares how database migrations must be done:
type User struct {
gorm.Model
Name string
Age sql.NullInt64
Birthday *time.Time
Email string `gorm:"type:varchar(100);unique_index"`
Role string `gorm:"size:255"` // set field size to 255
MemberNumber *string `gorm:"unique;not null"` // set member number to unique and not null
Num int `gorm:"AUTO_INCREMENT"` // set num to auto incrementable
Address string `gorm:"index:addr"` // create index with name `addr` for address
IgnoreMe int `gorm:"-"` // ignore this field
}
In this example for the field Email with gorm tag we declare that corresponding column in database for the field email must be of type varchar and 100 maximum length and it also must have unique index.
other example is binding tags that are used very mostly in gin package.
type Login struct {
User string `form:"user" json:"user" xml:"user" binding:"required"`
Password string `form:"password" json:"password" xml:"password" binding:"required"`
}
var json Login
if err := c.ShouldBindJSON(&json); err != nil {
c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
return
}
the binding tag in this example gives hint to gin package that the data sent to API must have user and password fields cause these fields are tagged as required.
So generraly tags are data that packages require to know how should they treat with data of type different structs and best way to get familiar with the tags a package needs is READING A PACKAGE DOCUMENTATION COMPLETELY.