我有一个环回应用程序，我希望能够在API调用中包含关系的计算属性。例如，假设我有apartment模型和address模型。 address具有属性city和state。
我想打电话给公寓模型，并将城市和州作为相关address模型中的单个字符串包含在内。
我从@Raymond Feng对this question的回答中获得了一些灵感，并尝试了以下方法（原谅coffeescript /伪代码）：
address.defineProperty(address.prototype, "fullAddress",
get: () -> return address.city + " " + address.state
)
然而，当我尝试：
apartment.findOne({
include:
relation: "address"
scope:
fields:
fullAddress: true
}, (err, apartment) ->
console.log(apartment)
)
我得到了
Error: ER_BAD_FIELD_ERROR: Unknown column 'fullAddress' in 'field list'
值得注意的是，当我尝试在不指定字段的情况下查询地址模型时，我得到一个名为“[object Object]”的属性，其值为null，我怀疑这是我尝试定义{{1}的结果} property。
我认为我用错误的语法来解决问题。我正在寻找的是什么，如果是的话，我该怎么做？
答案 0 :(得分：2)
依赖于相关模型的Loopback lacks out of the box support for computed properties，因为相关模型是异步加载的。但是，我写了一个解决这个问题的解决方案（原谅coffeescript）：
app.wrapper = (model, fn, args)->
deferred = Q.defer()
args.push((err, result)->
console.log(err) if err
throw err if err
deferred.resolve(result)
)
app.models[model][fn].apply(app.models[model], args)
return deferred.promise
app.mixCalcs = (model, fn, args)->
mainDeferred = Q.defer()
iterationDeferreds = new Array()
mixinCalcs = (model, relationHash) ->
#iterate if there if the model includes relations
if relationHash.scope? and relationHash.scope.include?
#test if hash includes multiple relations
if typeof relationHash.scope.include == "array"
_.each(relationHash.scope.include, (subRelationHash) ->
mixinCalcs(model[subRelationHash.relation](), subRelationHash)
)
else
mixinCalcs(model[relationHash.scope.include.relation](), relationHash.scope.include)
#iterate if the model to be unpacked is an array (toMany relationship)
if model[0]?
_.each(model, (subModel) ->
mixinCalcs(subModel, relationHash)
)
#we're done with this model, we don't want to mix anything into it
return
#check if the hash requests the inclusion of calcs
if relationHash.scope? and relationHash.scope.calc?
#setup deferreds because we will be loading things
iterationDeferred = Q.defer()
iterationDeferreds.push(iterationDeferred.promise)
calc = relationHash.scope.calc
#get the calcHash definition
calcHash = app.models[model.constructor.definition.name]["calcHash"]
#here we use a pair of deferreds. Inner deferrds load the reiquirements for each calculated val
#outer deferreds fire once all inner deferred deps are loaded to caluclate each val
#once all vals are calced the iteration deferred fires, resolving this object in the query
#once all iteration deferreds fire, we can send back the query through main deferred
outerDeferreds = new Array()
for k, v of calcHash
if calc[k]
((k, v) ->
outerDeferred = Q.defer()
outerDeferreds.push(outerDeferred.promise)
innerDeferreds = new Array()
#load each required relation, then resolve the inner promise
_.each(v.required, (req) ->
innerDeferred = Q.defer()
innerDeferreds.push(innerDeferred.promise)
model[req]((err, val) ->
console.log("inner Deferred for #{req} of #{model.constructor.definition.name}")
innerDeferred.resolve(val)
)
)
#all relations loaded, calculate the value and return it through outer deferred
Q.all(innerDeferreds).done((deps)->
ret = {}
ret[k] = v.fn(model, deps)
console.log("outer Deferred for #{k} of #{model.constructor.definition.name}")
outerDeferred.resolve(ret)
)
)(k, v)
#all calculations complete, mix them into the model
Q.all(outerDeferreds).done((deps)->
_.each(deps, (dep)->
for k, v of dep
model[k] = v
)
console.log("iteration Deferred for #{model.constructor.definition.name}")
iterationDeferred.resolve()
)
#/end iterate()
app.wrapper(model, fn, args).done((model) ->
mixinCalcs(model, {scope: args[0]})
console.log(iterationDeferreds)
#all models have been completed
Q.all(iterationDeferreds).done(()->
console.log("main Deferred")
mainDeferred.resolve(model)
)
)
return mainDeferred.promise
编译的Javascript（没有评论）：
app.wrapper = function(model, fn, args) {
var deferred;
deferred = Q.defer();
args.push(function(err, result) {
if (err) {
console.log(err);
}
if (err) {
throw err;
}
return deferred.resolve(result);
});
app.models[model][fn].apply(app.models[model], args);
return deferred.promise;
};
app.mixCalcs = function(model, fn, args) {
var iterationDeferreds, mainDeferred, mixinCalcs;
mainDeferred = Q.defer();
iterationDeferreds = new Array();
mixinCalcs = function(model, relationHash) {
var calc, calcHash, iterationDeferred, k, outerDeferreds, v;
if ((relationHash.scope != null) && (relationHash.scope.include != null)) {
if (typeof relationHash.scope.include === "array") {
_.each(relationHash.scope.include, function(subRelationHash) {
return mixinCalcs(model[subRelationHash.relation](), subRelationHash);
});
} else {
mixinCalcs(model[relationHash.scope.include.relation](), relationHash.scope.include);
}
}
if (model[0] != null) {
_.each(model, function(subModel) {
return mixinCalcs(subModel, relationHash);
});
return;
}
if ((relationHash.scope != null) && (relationHash.scope.calc != null)) {
iterationDeferred = Q.defer();
iterationDeferreds.push(iterationDeferred.promise);
calc = relationHash.scope.calc;
calcHash = app.models[model.constructor.definition.name]["calcHash"];
outerDeferreds = new Array();
for (k in calcHash) {
v = calcHash[k];
if (calc[k]) {
(function(k, v) {
var innerDeferreds, outerDeferred;
outerDeferred = Q.defer();
outerDeferreds.push(outerDeferred.promise);
innerDeferreds = new Array();
_.each(v.required, function(req) {
var innerDeferred;
innerDeferred = Q.defer();
innerDeferreds.push(innerDeferred.promise);
return model[req](function(err, val) {
console.log("inner Deferred for " + req + " of " + model.constructor.definition.name);
return innerDeferred.resolve(val);
});
});
return Q.all(innerDeferreds).done(function(deps) {
var ret;
ret = {};
ret[k] = v.fn(model, deps);
console.log("outer Deferred for " + k + " of " + model.constructor.definition.name);
return outerDeferred.resolve(ret);
});
})(k, v);
}
}
return Q.all(outerDeferreds).done(function(deps) {
_.each(deps, function(dep) {
var _results;
_results = [];
for (k in dep) {
v = dep[k];
_results.push(model[k] = v);
}
return _results;
});
console.log("iteration Deferred for " + model.constructor.definition.name);
return iterationDeferred.resolve();
});
}
};
app.wrapper(model, fn, args).done(function(model) {
mixinCalcs(model, {
scope: args[0]
});
console.log(iterationDeferreds);
return Q.all(iterationDeferreds).done(function() {
console.log("main Deferred");
return mainDeferred.resolve(model);
});
});
return mainDeferred.promise;
};
该插件依赖于Q和下划线，因此您需要包含这些库。上面的主要代码应该加载到bootscript中。使用以下语法在模型的js定义文件中定义计算属性：
MODEL_NAME.calcHash = {
"ATTRIBUTE_NAME":
required: ["REQUIRED", "RELATION", "MODEL", "NAMES"]
fn: (model, deps) ->
#function which should return the calculated value. Loaded relations are provided as an array to the deps arg
return deps[0].value + deps[1].value + deps[2].value
"ATTRIBUTE_TWO":
#...
}
使用以下语法调用插件：
app.mixCalcs("MODEL_NAME", "FUNCTION_NAME (i.e. 'findOne')", [arguments for the called function])
您的过滤器现在支持属性calc，其功能与fields类似，不同之处在于它将包含来自calcHash的计算属性。
使用示例：
query = Candidate.app.mixCalcs("Candidate", "findOne", [{
where:
id: 1
include:
relation: "user"
scope:
calc:
timeSinceLastLogin: true
calc:
fullName: true
}])
query.done((result)->
cb(null, result)
)
如果环回团队中的某个人可以将这些行中的功能合并到主要版本中，那就太棒了。我也是opened a loopback issue。
答案 1 :(得分：1)
Loopback不支持计算属性是不正确的。
这可以使用我在此处描述的Loopback＆＃39; operational hooks来完成：Dynamic Properties or Aggregate Functions in Loopback Models
答案 2 :(得分：0)
你可以尝试一些很棒的混音，这是我的收藏：
您可以尝试第三方插件：
1）用于聚合的环回连接器： https://github.com/benkroeger/loopback-connector-aggregate
2）用于计算/计算属性的环回混合（仅在创建新模型实例时有效）： https://github.com/fullcube/loopback-ds-calculated-mixin https://github.com/fullcube/loopback-ds-computed-mixin
3）Loopback mixin用于更改跟踪（每次更新时启动）： https://github.com/fullcube/loopback-ds-changed-mixin
4）如果您需要统计数据 - 这是另一个混音： https://github.com/jonathan-casarrubias/loopback-stats-mixin
5）您可以计算相关模型： https://github.com/exromany/loopback-counts-mixin
6）您可以自动反规范化并保存相关数据，并选择要存储的字段（对缓存很有用）： https://github.com/jbmarchetti/loopback-denormalize
7）如果在导入期间需要用于字段映射的计算属性： https://github.com/jonathan-casarrubias/loopback-import-mixin