【徹底解説】Angularで発生する「Can't construct a query for the property」エラーの原因と解決策

"Can't construct a query for the property, since the query selector wasn't defined" エラーは、Angular で @ViewChild や ContentChild などのデコレータを使用してコンポーネント内の要素にアクセスしようとしたときに発生します。このエラーは、以下のいずれかの理由で発生します。

解決策

このエラーを解決するには、以下の手順を試してください。

補足

このエラーは、Angular 8 以降で発生する可能性があります。Angular 7 以前では、このエラーは発生しませんでした。

このエラーは、コンポーネント間の通信や要素の操作に影響を与える可能性があります。問題を迅速に解決するために、上記の手順を順を追って実行することをお勧めします。

import { Component, ContentChild } from '@angular/core';

@Component({
  selector: 'app-parent',
  template: `
    <app-child></app-child>
  `,
})
export class ParentComponent {
  @ContentChild('my-child') childComponent: ChildComponent;
}

@Component({
  selector: 'app-child',
  template: `
    <p>I am a child component.</p>
  `,
})
export class ChildComponent {
}

In this example, the ParentComponent has a @ContentChild decorator that is trying to access a ChildComponent element with the selector my-child. However, the ChildComponent does not have a selector property defined, which is causing the error.

To fix the error, you need to add a selector property to the ChildComponent class:

@Component({
  selector: 'my-child',
  template: `
    <p>I am a child component.</p>
  `,
})
export class ChildComponent {
}

With this change, the ParentComponent should be able to access the ChildComponent element without any problems.

Here is another example of code that can cause the error:

import { Component, ViewChild } from '@angular/core';

@Component({
  selector: 'app-parent',
  template: `
    <app-child #myChild></app-child>
    <button (click)="accessChild()">Access Child</button>
  `,
})
export class ParentComponent {
  @ViewChild('myChild') childComponent: ChildComponent;

  accessChild() {
    console.log(this.childComponent);
  }
}

@Component({
  selector: 'app-child',
  template: `
    <p>I am a child component.</p>
  `,
})
export class ChildComponent {
}

@Component({
  selector: 'my-child',
  template: `
    <p>I am a child component.</p>
  `,
})
export class ChildComponent {
}

I hope this helps! Let me know if you have any other questions.

Use ContentChildren instead of ContentChild

If you need to access multiple child components of the same type, you can use the ContentChildren decorator instead of ContentChild. This decorator will return a QueryList of all the child components that match the selector.

Here is an example of how to use ContentChildren:

import { Component, ContentChildren } from '@angular/core';

@Component({
  selector: 'app-parent',
  template: `
    <ng-content select="app-child"></ng-content>
  `,
})
export class ParentComponent {
  @ContentChildren(ChildComponent) childComponents: QueryList<ChildComponent>;

  ngAfterViewInit() {
    console.log(this.childComponents);
  }
}

@Component({
  selector: 'app-child',
  template: `
    <p>I am a child component.</p>
  `,
})
export class ChildComponent {
}

Use @Input to pass data from child to parent

If you only need to access specific data from the child component, you can use the @Input decorator to pass the data from the child to the parent. This will allow you to access the data without having to use a query selector.

import { Component, Input } from '@angular/core';

@Component({
  selector: 'app-child',
  template: `
    <p>Child data: {{childData}}</p>
  `,
})
export class ChildComponent {
  @Input() childData: string;
}

@Component({
  selector: 'app-parent',
  template: `
    <app-child [childData]="parentData"></app-child>
  `,
})
export class ParentComponent {
  parentData = 'Hello from parent!';
}

In this example, the ChildComponent has an @Input property called childData. The ParentComponent sets the childData property to the value 'Hello from parent!' when it creates the ChildComponent instance. This allows the ChildComponent to access the data from the parent.

Use EventEmitter to communicate between components

If you need to communicate more complex data or events between components, you can use EventEmitter. This allows you to emit events from the child component that the parent component can listen for.

import { Component, Input, Output, EventEmitter } from '@angular/core';

@Component({
  selector: 'app-child',
  template: `
    <button (click)="onClick()">Click me</button>
  `,
})
export class ChildComponent {
  @Input() childData: string;

  @Output() onClick = new EventEmitter();

  handleClick() {
    this.onClick.emit();
  }
}

@Component({
  selector: 'app-parent',
  template: `
    <app-child [childData]="parentData" (onClick)="onChildClick()"></app-child>
  `,
})
export class ParentComponent {
  parentData = 'Hello from parent!';

  onChildClick() {
    console.log('Child clicked!');
  }
}

In this example, the ChildComponent has an @Output property called onClick that is an EventEmitter. The ParentComponent listens for the onClick event and calls the onChildClick() method when the event is emitted.

These are just a few of the ways to solve the "Can't construct a query for the property, since the query selector wasn't defined" error in Angular. The best approach for you will depend on your specific needs.