GoLang, REST, PATCH and building an UPDATE query

Question

since few days I was struggling on how to proceed with PATCH request in Go REST API until I have found an article about using pointers and omitempty tag which I have populated and is working fine. Fine until I have realized I still have to build an UPDATE SQL query.

My struct looks like this:

type Resource struct {
    Name        *string `json:"name,omitempty"        sql:"resource_id"`
    Description *string `json:"description,omitempty" sql:"description"`
}

I am expecting a PATCH /resources/{resource-id} request containing such a request body:

{"description":"Some new description"}

In my handler I will build the Resource object this way (ignoring imports, ignoring error handling):

var resource Resource
resourceID, _ := mux.Vars(r)["resource-id"]

d := json.NewDecoder(r.Body)
d.Decode(&resource)

// at this point our resource object should only contain
// the Description field with the value from JSON in request body

Now, for normal UPDATE (PUT request) I would do this (simplified):

stmt, _ := db.Prepare(`UPDATE resources SET description = ?, name = ? WHERE resource_id = ?`)
res, _ := stmt.Exec(resource.Description, resource.Name, resourceID)

The problem with PATCH and omitempty tag is that the object might be missing multiple properties, thus I cannot just prepare a statement with hardcoded fields and placeholders... I will have to build it dynamically.

And here comes my question: how can I build such UPDATE query dynamically? In the best case I'd need some solution with identifying the set properties, getting their SQL field names (probably from the tags) and then I should be able to build the UPDATE query. I know I can use reflection to get the object properties but have no idea hot to get their sql tag name and of course I'd like to avoid using reflection here if possible... Or I could simply check for each property it is not nil, but in real life the structs are much bigger than provided example here...

Can somebody help me with this one? Did somebody already have to solve the same/similar situation?

SOLUTION:

Based on the answers here I was able to come up with this abstract solution. The SQLPatches method builds the SQLPatch struct from the given struct (so no concrete struct specific):

import (
    "fmt"
    "encoding/json"
    "reflect"
    "strings"
)

const tagname = "sql"

type SQLPatch struct {
    Fields []string
    Args   []interface{}
}

func SQLPatches(resource interface{}) SQLPatch {
    var sqlPatch SQLPatch
    rType := reflect.TypeOf(resource)
    rVal := reflect.ValueOf(resource)
    n := rType.NumField()

    sqlPatch.Fields = make([]string, 0, n)
    sqlPatch.Args = make([]interface{}, 0, n)

    for i := 0; i < n; i++ {
        fType := rType.Field(i)
        fVal := rVal.Field(i)
        tag := fType.Tag.Get(tagname)

        // skip nil properties (not going to be patched), skip unexported fields, skip fields to be skipped for SQL
        if fVal.IsNil() || fType.PkgPath != "" || tag == "-" {
            continue
        }

        // if no tag is set, use the field name
        if tag == "" {
            tag = fType.Name
        }
        // and make the tag lowercase in the end
        tag = strings.ToLower(tag)

        sqlPatch.Fields = append(sqlPatch.Fields, tag+" = ?")

        var val reflect.Value
        if fVal.Kind() == reflect.Ptr {
            val = fVal.Elem()
        } else {
            val = fVal
        }

        switch val.Kind() {
        case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
            sqlPatch.Args = append(sqlPatch.Args, val.Int())
        case reflect.String:
            sqlPatch.Args = append(sqlPatch.Args, val.String())
        case reflect.Bool:
            if val.Bool() {
                sqlPatch.Args = append(sqlPatch.Args, 1)
            } else {
                sqlPatch.Args = append(sqlPatch.Args, 0)
            }
        }
    }

    return sqlPatch
}

Then I can simply call it like this:

type Resource struct {
    Description *string `json:"description,omitempty"`
    Name *string `json:"name,omitempty"`
}

func main() {
    var r Resource

    json.Unmarshal([]byte(`{"description": "new description"}`), &r)
    sqlPatch := SQLPatches(r)

    data, _ := json.Marshal(sqlPatch)
    fmt.Printf("%s\n", data)
}

You can check it at Go Playground. The only problem here I see is that I allocate both the slices with the amount of fields in the passed struct, which may be 10, even though I might only want to patch one property in the end resulting in allocating more memory than needed... Any idea how to avoid this?

Answer 1

I recently had same problem. about PATCH and looking around found this article. It also makes references to the RFC 5789 where it says:

The difference between the PUT and PATCH requests is reflected in the way the server processes the enclosed entity to modify the resource identified by the Request-URI. In a PUT request, the enclosed entity is considered to be a modified version of the resource stored on the origin server, and the client is requesting that the stored version be replaced. With PATCH, however, the enclosed entity contains a set of instructions describing how a resource currently residing on the origin server should be modified to produce a new version. The PATCH method affects the resource identified by the Request-URI, and it also MAY have side effects on other resources; i.e., new resources may be created, or existing ones modified, by the application of a PATCH.

e.g:

[
    { "op": "test", "path": "/a/b/c", "value": "foo" },
    { "op": "remove", "path": "/a/b/c" },
    { "op": "add", "path": "/a/b/c", "value": [ "foo", "bar" ] },
    { "op": "replace", "path": "/a/b/c", "value": 42 },
    { "op": "move", "from": "/a/b/c", "path": "/a/b/d" },
    { "op": "copy", "from": "/a/b/d", "path": "/a/b/e" }
]

This set of instructions should make it easier to build the update query.

EDIT

This is how you would obtain sql tags but you will have to use reflection:

type Resource struct {
        Name        *string `json:"name,omitempty"        sql:"resource_id"`
        Description *string `json:"description,omitempty" sql:"description"`
}

sp := "sort of string"
r := Resource{Description: &sp}
rt := reflect.TypeOf(r) // reflect.Type
rv := reflect.ValueOf(r) // reflect.Value

for i := 0; i < rv.NumField(); i++ { // Iterate over all the fields
    if !rv.Field(i).IsNil() { // Check it is not nil

        // Here you would do what you want to having the sql tag.
        // Creating the query would be easy, however
        // not sure you would execute the statement

        fmt.Println(rt.Field(i).Tag.Get("sql")) // Output: description
    }
}   

I understand you don't want to use reflection, but still this may be a better answer than the previous one as you comment state.

EDIT 2:

About the allocation - read this guide lines of Effective Go about Data structures and Allocation:

// Here you are allocating an slice of 0 length with a capacity of n
sqlPatch.Fields = make([]string, 0, n)
sqlPatch.Args = make([]interface{}, 0, n)

With make(Type, Length, Capacity (optional))

Consider the following example:

// newly allocated zeroed value with Composite Literal 
// length: 0
// capacity: 0
testSlice := []int{}
fmt.Println(len(testSlice), cap(testSlice)) // 0 0
fmt.Println(testSlice) // []

// newly allocated non zeroed value with make   
// length: 0
// capacity: 10
testSlice = make([]int, 0, 10)
fmt.Println(len(testSlice), cap(testSlice)) // 0 10
fmt.Println(testSlice) // []

// newly allocated non zeroed value with make   
// length: 2
// capacity: 4
testSlice = make([]int, 2, 4)
fmt.Println(len(testSlice), cap(testSlice)) // 2 4
fmt.Println(testSlice) // [0 0]

In your case, may want to the following:

// Replace this
sqlPatch.Fields = make([]string, 0, n)
sqlPatch.Args = make([]interface{}, 0, n)

// With this or simple omit the capacity in make above
sqlPatch.Fields = []string{}
sqlPatch.Args = []interface{}{}

// The allocation will go as follow: length - capacity
testSlice := []int{} // 0 - 0
testSlice = append(testSlice, 1) // 1 - 2
testSlice = append(testSlice, 1) // 2 - 2   
testSlice = append(testSlice, 1) // 3 - 4   
testSlice = append(testSlice, 1) // 4 - 4   
testSlice = append(testSlice, 1) // 5 - 8

Answer 2

Alright, I think the solution I used back in 2016 was quite over-engineered for even more over-engineered problem and was completely unnecessary. The question asked here was very generalized, however we were building a solution that was able to build its SQL query on its own and based on the JSON object or query parameters and/or Headers sent in the request. And that to be as generic as possible.

Nowadays I think the best solution is to avoid PATCH unless truly necessary. And even then you still can use PUT and replace the whole resource with patched property/ies coming already from the client - i.e. not giving the client the option/possibility to send any PATCH request to your server and to deal with partial updates on their own.

However this is not always recommended, especially in cases of bigger objects to save some C0² by reducing the amount of redundant transmitted data. Whenever today if I need to enable a PATCH for the client I simply define what can be patched - this gives me clarity and the final struct.

Note that I am using a IETF documented JSON Merge Patch implementation. I consider that of JSON Patch (also documented by IETF) redundant as hypothetically we could replace the whole REST API by having one single JSON Patch endpoint and let clients control the resources via allowed operations. I also think the implementation of such JSON Patch on the server side is way more complicated. The only use-case I could think of using such implementation is if I was implementing a REST API over a file system...

So the struct may be defined as in my OP:

    type ResourcePatch struct {
        ResourceID  some.UUID `json:"resource_id"`
        Description *string `json:"description,omitempty"`
        Name        *string `json:"name,omitempty"`
    }

In the handler func I'd decode the ID from the path into the ResourcePatch instance and unmarshall JSON from the request body into it, too.

Sending only this

{"description":"Some new description"}

to PATCH /resources/<UUID>

I should end up with with this object:

ResourcePatch
    * ResourceID {"UUID"}
    * Description {"Some new description"}

And now the magic: use a simple logic to build the query and exec parameters. For some it may seem tedious or repetitive or unclean for bigger PATCH objects, but my reply to this would be: if your PATCH object consists of more than 50% of the original resource' properties (or simply too many for your liking) use PUT and expect the clients to send (and replace) the whole resource instead.

It could look like this:

    func (s Store) patchMyResource(r models.ResourcePatch) error {
        q := `UPDATE resources SET `
        qParts := make([]string, 0, 2)
        args := make([]interface{}, 0, 2)

        if r.Description != nil {
            qParts = append(qParts, `description = ?`)
            args = append(args, r.Description)
        }

        if r.Name != nil {
            qParts = append(qParts, `name = ?`)
            args = append(args, r.Name)
        }

        q += strings.Join(qParts, ',') + ` WHERE resource_id = ?`
        args = append(args, r.ResourceID)

        _, err := s.db.Exec(q, args...)

        return err
    }

I think there's nothing simpler and more effective. No reflection, no over-kills, reads quite good.

Answer 3

Struct tags are only visible through reflection, sorry.

If you don't want to use reflection (or, I think, even if you do), I think it is Go-like to define a function or method that "marshals" your struct into something that can easily be turned into a comma-separated list of SQL updates, and then use that. Build small things to help solve your problems.

For example given:

type Resource struct {
    Name        *string `json:"name,omitempty"        sql:"resource_id"`
    Description *string `json:"description,omitempty" sql:"description"`
}

You might define:

func (r Resource) SQLUpdates() SQLUpdates {
    var s SQLUpdates
    if (r.Name != nil) {
        s.add("resource_id", *r.Name)
    }
    if (r.Description != nil) {
        s.add("description", *r.Description)
    }
}

where the type SQLUpdates looks something like this:

type SQLUpdates struct {
    assignments []string
    values []interface{}
}
func (s *SQLUpdates) add(key string, value interface{}) {
    if (s.assignments == nil) {
        s.assignments = make([]string, 0, 1)
    }
    if (s.values == nil) {
        s.values = make([]interface{}, 0, 1)
    }
    s.assignments = append(s.assignments, fmt.Sprintf("%s = ?", key))
    s.values = append(s.values, value)
}
func (s SQLUpdates) Assignments() string {
    return strings.Join(s.assignments, ", ")
}
func (s SQLUpdates) Values() []interface{} {
    return s.values
}

See it working (sorta) here: https://play.golang.org/p/IQAHgqfBRh

If you have deep structs-within-structs, it should be fairly easy to build on this. And if you change to an SQL engine that allows or encourages positional arguments like $1 instead of ?, it's easy to add that behavior to just the SQLUpdates struct without changing any code that used it.

For the purpose of getting arguments to pass to Exec, you would just expand the output of Values() with the ... operator.