How to join two tables in GORM - go

Question

I have two tables,
SET search_path = public;
CREATE TABLE IF NOT EXISTS changelog
(
id BIGINT NOT NULL PRIMARY KEY,
object_type TEXT,
object_id BIGINT,
parent_type TEXT,
parent_id BIGINT,
action TEXT,
field TEXT,
old_value TEXT,
new_value TEXT,
comment_id INTEGER,
created_on TIMESTAMP WITHOUT TIME ZONE,
created_by BIGINT
);
CREATE TABLE IF NOT EXISTS changelog_comments
(
id INTEGER NOT NULL PRIMARY KEY,
comment TEXT,
created_on TIMESTAMP WITHOUT TIME ZONE,
created_by BIGINT
);
SET search_path = DEFAULT;
I want to implement a search method for the changelog which returns the fields
"objectType"
"objectId"
"parentType"
"parentId"
"action"
"field"
"oldValue"
"newValue"
"comment"
"createdBy"
"createdOn"
as you can see the result comes from the join of the two tables.
I found https://gorm.io/docs/preload.html but to be honest, didn't get that how can I achieve what I need.
I thought something like the following could be helpful
type ChangelogResponseItem struct {
ObjectType string `json:"objectType"`
ObjectID uuid.UUID `json:"objectId"`
ParentType string `json:"parentType"`
ParentID uuid.UUID `json:"parentId"`
Action string `json:"action"`
Field *string `json:"field"`
OldValue *string `json:"oldValue"`
NewValue *string `json:"newValue"`
Comment *string `json:"comment"`
CreatedBy *uint64 `json:"createdBy"`
CreatedOn *time.Time `json:"createdOn"`
}
The question is that how can get what I mentioned from the mentioned tables in GORM?

Answer 1

One way to do it would be to combine Joins and Select methods to get what you want. Based on your table, it would look something like this:
list := []ChangelogResponseItem{}
tx := db.Table("changelog").
Joins("INNER JOIN changelog_comments cc ON cc.id = changelog.comment_id").
Select("changelog.objectType, changelog.object_type, changelog.object_id, changelog.parent_type, changelog.parent_id, changelog.action, changelog.field, changelog.old_value, changelog.new_value, cc.comment, changelog.created_on, changelog.created_by").
Find(&list)
if tx.Error != nil {
// handle error
}
This is just to return the data, a search would include additional Where methods.
EDIT:
Solution with a preload option:
Structs:
type ChangelogComment struct {
ID uint64 `json:"id"`
Comment string `json:"comment"`
}
type Changelog struct {
ObjectType string `json:"objectType"`
ObjectID uuid.UUID `json:"objectId"`
ParentType string `json:"parentType"`
ParentID uuid.UUID `json:"parentId"`
Action string `json:"action"`
Field *string `json:"field"`
OldValue *string `json:"oldValue"`
NewValue *string `json:"newValue"`
CommentID uint64 `json:"comment_id"`
Comment *ChangelogComment `json:"comment"`
CreatedBy *uint64 `json:"createdBy"`
CreatedOn *time.Time `json:"createdOn"`
}
Code with the Preload method:
list := []Changelog{}
tx := db.Preload("Comment").Find(&list)
if tx.Error != nil {
// handle error
}
Please note that in this case, you will have a different JSON object, the structure od the object will not be flat, because you will have a comment field as well.