How to Effectively Mock gin.Context's BindJSON for Go Testing?

Setting Mock gin.Context for BindJSON in Go

Introduction

Binding JSON data to a struct during HTTP requests is a common task in web applications. It can become challenging when utilizing a testing framework that requires mocking the request context. Specifically, mocking gin.Context poses difficulties when attempting to test functions that rely on its BindJSON method. This article provides a comprehensive solution to this issue.

Mocking gin.Context

Firstly, it is crucial to instantiate a test gin.Context and set its http.Request as non-null:

    w := httptest.NewRecorder()
    c, _ := gin.CreateTestContext(w) 

    c.Request = &http.Request{
        Header: make(http.Header),
    }

Mocking POST JSON Body

Next, we can mock a POST JSON body using the following function:

func MockJsonPost(c *gin.Context /* the test context */, content interface{}) {
    c.Request.Method = "POST" // or PUT
    c.Request.Header.Set("Content-Type", "application/json")

    jsonbytes, err := json.Marshal(content)
    if err != nil {
        panic(err)
    }
    
    // the request body must be an io.ReadCloser
    // the bytes buffer though doesn't implement io.Closer,
    // so you wrap it in a no-op closer
    c.Request.Body = io.NopCloser(bytes.NewBuffer(jsonbytes))
}

This function accepts a content interface{} parameter that can be marshalled into JSON using json.Marshal(). This could be a struct with appropriate JSON tags or a map[string]interface{}.

Example Usage

Here's how to use the MockJsonPost function in a test:

func TestMyHandler(t *testing.T) {
    w := httptest.NewRecorder()
    ctx, _ := gin.CreateTestContext(w) 

    ctx.Request = &http.Request{
        Header: make(http.Header),
    }


    MockJsonPost(ctx, map[string]interface{}{"foo": "bar"})
    
    MyHandler(ctx)
    assert.EqualValues(t, http.StatusOK, w.Code)
} 

Additional Notes

For further information on testing Gin handlers, refer to the following resource:

• How to unit test a Go Gin handler function?

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