How to Handle MongoDB Updates with Golang\'s `omitempty` Flag?

Updating MongoDB Fields with Omitempty Flag in Golang Structures

The omitempty flag in Golang structures allows developers to exclude fields from JSON marshalling if they have zero values. However, this behavior can pose challenges when updating MongoDB documents.

Consider a Coupon form where some fields are optional. A Golang structure representing the form might have omitempty flags on these fields like:

type Coupon struct {
    Id               int    `json:"id,omitempty" bson:"_id,omitempty"`
    Name             string `json:"name,omitempty" bson:"name,omitempty"`
    Code             string `json:"code,omitempty" bson:"code,omitempty"`
    Description      string `json:"description,omitempty" bson:"description,omitempty"`
    Status           bool   `json:"status" bson:"status"`
    MaxUsageLimit    int    `json:"max_usage_limit,omitempty" bson:"max_usage_limit,omitempty"`
    SingleUsePerUser bool   `json:"single_use_per_user,omitempty" bson:"single_use_per_user,omitempty"`
}

Problem

The issue arises when updating the form. If a previously checked checkbox (bool field) is unchecked during form submission, the omitempty flag excludes it from the structure. Consequently, the value is not updated in the MongoDB document.

Similarly, if only required fields are provided in a REST API request, MongoDB overwrites the entire document, including values that should not be updated.

Solution

To overcome this issue, it is necessary to change the fields annotated with omitempty to pointers. This allows the fields to have a nil value, which represents the "not updated" state:

type Coupon struct {
    Id               *int    `json:"id,omitempty" bson:"_id,omitempty"`
    Name             string `json:"name,omitempty" bson:"name,omitempty"`
    Code             string `json:"code,omitempty" bson:"code,omitempty"`
    Description      string `json:"description,omitempty" bson:"description,omitempty"`
    Status           *bool   `json:"status" bson:"status"`
    MaxUsageLimit    *int    `json:"max_usage_limit,omitempty" bson:"max_usage_limit,omitempty"`
    SingleUsePerUser *bool   `json:"single_use_per_user,omitempty" bson:"single_use_per_user,omitempty"`
}

With this modification, a nil pointer indicates that the field should not be updated. If a non-nil pointer is provided, its value will be set in the MongoDB document. This effectively resolves the issue with updating bool and int fields while retaining the omitempty flag.

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