如何在插入HTML5 Javascript sqlite上的表之前检查数据是否已存在？ [英] How to check if data already exists before inserting on a table on HTML5 Javascript sqlite?

问题描述

我正在开发一个Web应用程序，它使用HTML5 / Javascript sqlite抓取网站RSS提要并将其保存到数据库中。

I'm developing a web application that grabs a website RSS feed and save it into a database, using HTML5/Javascript sqlite.

这是我的代码

function createPosts(title,author,content,contentSnippet,publishedDate,img_destaque){
        var oldPost = checkIfExists(publishedDate);
        console.log(oldPost);
        if(!oldPost){
            var query = "insert into mytable (titulo, data, autor, img_destaque,descricao,postagem) VALUES (?, ?, ?, ?, ?, ?);";
            try {
                localDB.transaction(function(transaction){
                    transaction.executeSql(query, [title, publishedDate, author, img_destaque, contentSnippet, content], function(transaction, results){
                        if (!results.rowsAffected) {
                            updateStatus("Error, not data inserted");
                        }
                        else {
                            updateStatus("Data inserted, id: " + results.insertId);
                        }
                    }, errorHandler);
                });
            } 
            catch (e) {
                updateStatus("Error: " + e + ".");
            }
        }
        else{
            updateStatus("Error: Post already exists");
        }

}

这是函数checkIfExists

And this is the function checkIfExists

function checkIfExists(publishedDate){
    var query = "SELECT * FROM mytable;";
    var returnStatus;
    try {
        localDB.transaction(function(transaction){
            transaction.executeSql(query, [], function(transaction, results){
                for (var i = 0; i < results.rows.length; i++) {
                    var row = results.rows.item(i);
                    if(publishedDate === row.data){
                        console.log(publishedDate+' '+row.data);
                        returnStatus = true;
                    }
                }
                if(i == results.rows.length){
                    returnStatus = false;
                }
            }, function(transaction, result){
                    returnStatus = true;
            }, function(transaction, error){
                updateStatus("Erro: " + error.code + "<br>Mensagem: " + error.message);
                    returnStatus = true;
            });

        });
    } 
    catch (e) {
        updateStatus("Error: SELECT não realizado " + e + ".");
        returnStatus = true;
    }
    finally{
        console.log(returnStatus);
        return returnStatus;
    }
}

但它不起作用oldPost总是返回undefined和我的控制台日志记录，我看到在函数返回oldPost变量值后处理localDB.transaction。我如何解决这个问题？

But it's not working oldPost is always returning "undefined", and with my console logging I see that localDB.transaction is processed after the function returns the "oldPost" variable value. How do I solve this?

推荐答案

如何重构 checkifExists 两个回调并使用where子句到sql而不是在javascript中进行搜索所以

how about refactoring checkifExists to take two callbacks and use a where clause to the sql rather then doing the search in javascript so

function checkIfExists(publishedDate,callbacktrue,callbackfalse){
    var query = "SELECT * FROM mytable WHERE data=?;";
    try {
        localDB.transaction(function(transaction){
            transaction.executeSql(query, [publishedDate], function(transaction, results){
                if(results.rows.length > 0){
                    callbacktrue();
                } else {
                    callbackfalse();
                }
            }, function(transaction, result){
                    callbacktrue();
            }, function(transaction, error){
                updateStatus("Erro: " + error.code + "<br>Mensagem: " + error.message);
                    callbacktrue();
            });

        });
    } 
    catch (e) {
        updateStatus("Error: SELECT não realizado " + e + ".");
        callbacktrue();
    }
}
function createPosts(title,author,content,contentSnippet,publishedDate,img_destaque){
    checkIfExists(publishedDate,function(){
            updateStatus("Error: Post already exists");
        },function(){
            var query = "insert into mytable (titulo, data, autor, img_destaque,descricao,postagem) VALUES (?, ?, ?, ?, ?, ?);";
            try {
                localDB.transaction(function(transaction){
                    transaction.executeSql(query, [title, publishedDate, author, img_destaque, contentSnippet, content], function(transaction, results){
                        if (!results.rowsAffected) {
                            updateStatus("Error, not data inserted");
                        }
                        else {
                            updateStatus("Data inserted, id: " + results.insertId);
                        }
                    }, errorHandler);
                });
            } 
            catch (e) {
                updateStatus("Error: " + e + ".");
            }
    });               
}

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