I am trying to insert a struct to mongo. Firstly I get the data from an API as JSON and assign the data to a struct. Some fields might be nil. After that I insert the struct to mongoDB. So the problem I get is that when inserted, all the fields are initialized. For example I have a struct like this:
type VirtualMachine struct {
VirtualMachineID utils.SUUID `bson:"VirtualMachineID"`
Cdroms []*VM.VirtualMachineCdrom `bson:"Cdroms"`
CpuAllocatedMHz int `bson:"CpuAllocatedMHz"`
Name string `bson:"Name"`
}
If I get Json data like this
{
"VirtualMachineID":'16as4df663a',
"Cdroms":null,
"CpuAllocatedMHz":1666,
"Name":'VMName'
}
after I put it to mongo, the null field becomes an empty array. I need to avoid that. 'omitempty' did not help because it skips the field as well if the provided field happens to be an empty array and not null.
Firstly I thought it was because of the pointers, but later I found that the same happens to all data types. Shortly, if its nil, mgo converts it to its zero value.
I think I am missing something here, because it would be weird if mgo converts all nil values to their zero values by design.
Try *[]*VM.VirtualMachineCdrom (or just *[]VM.VirtualMachineCdrom if you don't actually need the elements to be pointers). A nil slice == a zero length slice, but a nil pointer to a slice does not.
Related
I am trying to do the following:
Deserialise a struct of fields into another struct of different format.
I am trying to do the transformation via an Hashmap as this seems to be the best suited way.
I am required to do this as a part of transformation of a legacy system, and this being one of the intermediary phases.
I have raised another question which caters to a subset of the same use-case, but do not think it gives enough overview, hence raising this question with more details and code.
(Rust converting a struct o key value pair where the field is not none)
Will be merging both questions once I figure out how.
Scenario:
I am getting input via an IPC through a huge proto.
I am using prost to deserialise this data into a struct.
My deserialised struct has n no of fields and can increase as well.
I need to transform this deserialised struct into a key,value struct. (shown ahead).
The incoming data, will mostly have a majority of null keys .i.e out of n fields, most likely only 1,2 have values at a given time.
Input struct after prost deserialisation:
Using proto3 so unable to define optional fields.
Ideally I would prefer a prost struct of options on every field. .i.e Option instead of string.
struct Data{
int32 field1,
int64 field2,
string field3,
...
}
This needs to be transformed to a genric struct as below for further use:
struct TransformedData
{
string Key
string Value
}
So,
Data{
field1: 20
field2: null
field3: null
field4: null
....
}
becomes
TransformedData{
key:"field1"
Value: "20"
}
Methods I have tried:
Method1
Add serde to the prost struct definiton and deserialise it into a map.
Loop over each item in a map to get values which are non-null.
Use these to create a struct
https://play.rust-lang.org/?version=stable&mode=debug&edition=2015&gist=9645b1158de31fd54976926c9665d6b4
This has its challenges:
Each primitve data type has its own default values and needs to be checked for.
Nested structs will result in object data type which needs to be handled.
Need to iterate over every field of a struct to determine non null values.
1&2 can be mitigated by setting an Optional field(yet to figure out how in proto3 and prost)
But I am not sure how to get over 3.
Iterating over a large struct to find non null fields is not scalable and will be expensive.
Method 2:
I am using prost reflects dynamic reflection to deserialise and get only specified value.
Doing this by ensuring each proto message has two extra fields:
proto -> signifying the proto struct being used when serializing.
key -> signifying the filed which has value when serializing.
let fd = package::FILE_DESCRIPTOR_SET;
let pool = DescriptorPool::decode(fd).unwrap();
let proto_message: package::GenericProto = prost::Message::decode(message.as_ref()).expect("error when de-serialising protobuf message to struct");
let proto = proto_message.proto.as_str();
let key = proto_message.key.as_str();
Then using key , I derive the key from what looks to be a map implemented by prost:
let message_descriptor = pool.get_message_by_name(proto).unwrap();
let dynamic_message = DynamicMessage::decode(message_descriptor, message.as_ref()).unwrap();
let data = dynamic_message.get_field_by_name(key).unwrap().as_ref().clone();
Here :
1.This fails when someone sends a struct with multiple fields filled.
2.This does not work for nested objects, as nested objects in turn needs to be converted to a map.
I am looking for the least expensive way to do the above.
I understand the solution I am looking for is pretty out there and I am taking a one size fits all approach.
Any pointers appreciated.
I am new to golang and I have one issue which I think community can help me to solve it.
I have one data structure like below
type ParentIDInfo struct {
PCOrderID string `json:"PCorderId",omitempty"`
TableVarieties TableVarietyDC `json:"tableVariety",omitempty"`
ProduceID string `json:"PRID",omitempty"`
}
type PCDCOrderAsset struct {
PcID string `json:"PCID",omitempty"`
DcID string `json:"DCID",omitempty"`
RequiredDate string `json:"requiredDate",omitempty"`
Qty uint64 `json:"QTY",omitempty"`
OrderID string `json:"ORDERID",omitempty"`
Status string `json:"STATUS",omitempty"`
Produce string `json:"Produce",omitempty"`
Variety string `json:"VARIETY",omitempty"`
Transports []TransportaionPCDC `json:"Transportaion",omitempty"`
ParentInfo []ParentIDInfo `json:"ParentInfo",omitempty"`
So I have issue to access the PCOrderID which inside the []ParentIDInfo . I have tried below however I getting error as "pcdcorder.ParentInfo.PCOrderID undefined (type []ParentIDInfo has no field or method PCOrderID)"
keyfarmercas = append(keyfarmercas, pcdcorder.ParentInfo.PCOrderID)
Any help will be very good
Thanks in advance
PCDCOrderAsset.ParentInfo is not a struct, it does not have a PCOrderID field. It's a slice (of element type ParentIDInfo), so its elements do, e.g. pcdcorder.ParentInfo[0].PCOrderID.
Whether this is what you want we can't tell. pcdcorder.ParentInfo[0].PCOrderID gives you the PCOrderID field of the first element of the slice. Based on your question this may or may not be what you want. You may want to append all IDs (one from each element). Also note that if the slice is empty (its length is 0), then pcdcorder.ParentInfo[0] would result in a runtime panic. You could avoid that by first checking its length and only index it if its not empty.
In case you'd want to add ids of all elements, you could use a for loop to do that, e.g.:
for i := range pcdorder.ParentInfo {
keyfarmercas = append(keyfarmercas, pcdcorder.ParentInfo[i].PCOrderID)
}
I am very new to Go and am trying to get my head around all the different types and how to use them. I have an interface with the following (which was originally in a json file):
[map[item:electricity transform:{fuelType}] map[transform:{fuelType} item:gas]]
and I have the following struct
type urlTransform struct {
item string
transform string
}
I have no idea how to get the interface data into the struct; I'm sure this is really stupid, but I have been trying all day. Any help would be greatly appreciated.
Decode the JSON directly to types you want instead of decoding to an interface{}.
Declare types that match the structure of your JSON data. Use structs for JSON objects and slices for JSON arrays:
type transform struct {
// not enough information in question to fill this in.
}
type urlTransform struct {
Item string
Transform transform
}
var transforms []urlTransform
The field names must be exported (start with uppercase letter).
Unmarshal the JSON to the declared value:
err := json.Unmarshal(data, &transforms)
or
err := json.NewDecoder(reader).Decode(&transforms)
From your response : [map[item:electricity transform:{fuelType}] map[transform:{fuelType} item:gas]].
As you can see here this is a an array that has map in it.
One way to get the value from this is :
values := yourResponse[0].(map[string]interface{}). // convert first index to map that has interface value.
transform := urlTransform{}
transform.Item = values["item"].(string) // convert the item value to string
transform.Transform = values["transform"].(string)
//and so on...
as you can see from the code above I'm getting the the value using map. And convert the value to the appropriate type in this case is string.
You can convert it to appropriate type like int or bool or other type. but this approach is painful as you need to get the value one bye one and assign it your your field struct.
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
Whats the correct way in go to distinguish between when a value in a struct was never set, or is just empty, for example, given the following:
type Organisation struct {
Category string
Code string
Name string
}
I need to know (for example) if the category was never set, or was saved as blank by the user, should I be doing this:
type Organisation struct {
Category *string
Code *string
Name *string
}
I also need to ensure I correctly persist either null or an empty string to the database
I'm still learning GO so it is entirely possible my question needs more info.
The zero value for a string is an empty string, and you can't distinguish between the two.
If you are using the database/sql package, and need to distinguish between NULL and empty strings, consider using the sql.NullString type. It is a simple struct that keeps track of the NULL state:
type NullString struct {
String string
Valid bool // Valid is true if String is not NULL
}
You can scan into this type and use it as a query parameter, and the package will handle the NULL state for you.
Google's protocol buffers (https://code.google.com/p/goprotobuf/) use pointers to describe optional fields.
The generated objects provide GetFoo methods which take the pain away from testing for nil (a.GetFoo() returns an empty string if a.Foo is nil, otherwise it returns *a.Foo).
It introduces a nuisance when you want to write literal structs (in tests, for example), because &"something" is not valid syntax to generate a pointer to a string, so you need a helper function (see, for example, the source code of the protocol buffer library for proto.String).
// String is a helper routine that allocates a new string value
// to store v and returns a pointer to it.
func String(v string) *string {
return &v
}
Overall, using pointers to represent optional fields is not without drawbacks, but it's certainly a viable design choice.
The standard database/sql package provides a NullString struct (members are just String string and Valid bool). To take care of some of the repetitive work of persistence, you could look at an object-relational manager like gorp.
I looked into whether there was some way to distinguish two kinds of empty string just out of curiosity, and couldn't find one. With []bytes, []byte{} == []byte(nil) currently returns false, but I'm not sure if the spec guarantees that to always remain true. In any case, it seems like the most practical thing to do is to go with the flow and use NullString.