Getting Start

This is a utility package of Golang provides convenient way to make conversion between maps and structs. There are two functions in this package:

func Map2Struct(m maps[string]interface{}, s interface{}) (error)

Map2Struct() converts a map to struct. You can custom how the fields of the struct match the keys in the map in a very simple way. You can also specify a default value or report an error if the key is not found.

func Struct2Map(s interface{}) map[string]interface{}

Struct2Map(), on the contrary, converts a struct into a map. This function is less powerful, honestly, but it's also helpful in some situations.

To be clear, we use the term 'field' to indicate the field in struct. And we use term 'key' and 'value' to indicate key and value in map.

Map2Struct

func Map2Struct(m maps[string]interface{}, s interface{}) (error)

This function converts a map into a struct. To use this function, import it first:

import Map2Struct from mapstruct

Here is an example:

m := map[string]interface{} {
	"Id"		: 1001,
	"Name"		: "tiaotiao",
}

s := struct {
	Id		int64		// 1001
	Name		string		// "tiaotiao"
}{}

err := Map2Struct(m, &s)

The type of map must be map[string]interface{}.

You can predefine the struct somewhere else, or just define a new type of struct where you need it.

The second parameter must be a reference of the struct, so that it can be modified by the function.

Map2Struct will go through all the fields of the struct one by one, to look up if is there any key in the map matches the field. If there is a key in the map matches, the function will assign the value to the field.

By default, Map2Struct uses the exact name of the field to search for keys. It is case sensitive.

If the type of value is not the same as the type of field, Map2Struct will try it best to convert and assign the value to the field. We will discuss it later.

If a field is not found, it will stay unchanged by default. You are able to assign it a default value or report an error by giving the field a tag.

Only public fields of the struct will be processed. It means fields starting with lowercase will be ignored.

Name Tags

In most case, we want to custom another names for fields. For instance, if the field of struct is 'Name' but we want to match the key 'user_name' in the map. We can achieve this by giving the field a tag.

struct {
	Id		int64	`map:"user_id"`		// search for "user_id"
	Name		string	`map:"user_name"`	// search for "user_name"
}

We use 'map' to indicate this tag is for our package. The following string within the quote is the customed name for the field which you want to search for in the map.

Struct tag is a feature of Golang. You may probably see other well known struct tags like 'xml' or 'json' somewhere else.

Ignore Field

struct {
	Website		string	`map:"-"`		// ignored
}

Here is a special field tag "-" which indicate this field will be ignored by Map2Struct. If a field tag is "-", this field will always stay unchanged after the function call, even if there is a key in the map matches this field exactly.

Default Value

We are able to assign the field a default value if it's not found in the map.

struct {
	Id		int64	`map:"user_id,-1"`		// default value is -1
	Name		string	`map:"user_name"`		// no default value
	Blocked		bool	`map:"blocked,false"`		// default value is false
}

Default value can be defined followed by the field tag, separated by a comma.

Default value only applies when field is not found.

Since default value is defined as type of string, the function will try it best to convert it to the target type. It will return an error if the conversion is failed.

Required

If the we set "required" as default value, it indicates this field is required from the map. Map2Struct will return an error if it is not found.

struct {
	Id	int64	`map:"user_id,required"`
	Name	string	`map:"user_name,required"`
}

This feature is really useful for me which simplifies my coding a lot.

Type Conversion

The following conversion rules apply to both map values and default values.

First of all, Map2Struct will try to assign or convert it directly using standard Golang library. This step should cover most of the situations.

If the first step failed, we expect that the value should be a string or json which can be interpreted to the target type. Otherwise it returns an error.

Map2Struct use different methods to interpret a string value, based on type of the target field.

• If the field is an integer, float or bool, we use strconv.ParseInt(), ParseUint(), ParseFloat(), ParseBool() to convert the string.
• If the field is a struct, pointer, slice, slice of objects, etc., we interpret the string as a json using json.Unmarshal(). The value type can be a string or json.RawMessage.

m := map[string]interface{}{
	"user_ids":		[]int64{501, 502, 503},
	"user_names":		`["batman", "ironman", "superman"]`,
	"group_info":		`{"id":1, "name":"heros"}`,
	"relative_groups":	json.RawMessage(`[{"id":7, "name":"FBI"}, {"id":9, "name":"CIA"}]`),
}

s := struct {
	Ids		[]int64		`map:"user_ids"`	// [501, 502, 503]
	Names		[]string	`map:"user_names"`	// ["batman", "ironman", "superman"]
	Group		group_info	`map:"group_info"`	// {1, "heros"}
	Relatives	[]*group_info	`map:"relative_groups"`	// [{7, "FBI"}, {9, "CIA"}]
}{}
	
err := Map2Struct(m, &s)

Struct2Map

func Struct2Map(s interface{}) map[string]interface{}

This function converts a struct to map. To use this function, import it first:

import Struct2Map from mapstruct

Here is an example:

s := struct {
	Id		int64
	Name		string
}{
	Id :		1001,
	Name :		"tiaotiao",
}

m := Struct2Map(s)
/*{
	"Id" :		1001,
	"Name":		"tiaotiao",
}*/

Struct2Map will create and return a new map from the given struct. The keys of the map will be the name of fields. The values will be the value of fields.

Only public fields will be processed. So fields starting with lowercase will be ignored.

Name Tags

struct {
	Id		int64	`map:"user_id"`
	Name		string	`map:"user_name"`
}

We can give the field a tag to specicy another name to be used as the key.

See Map2Struct above.

Ignore Field

struct {
	Other		string `map:"-"`
}

If we give the special tag "-" to a field, it will be ignored.

Omit Empty

struct {
	Description		string	`map:"desc,omitempty"`
}

If tag option is "omitempty", this field will not appear in the map if the value is empty.

Empty values are 0, false, "", nil, empty array and empty map.

To String

struct {
	Id		int64	`map:"id,string"`
	Price 		float32	`map:"price,string"`
}

If tag option is "string", this field will be convert to string type. Struct2Map will put the original value to the map if the conversion is failed.