Firebase query if child of child contains a value

The structure of the table is:

• chats
• --> randomId
• -->--> participants
• -->-->--> 0: 'name1'
• -->-->--> 1: 'name2'
• -->--> chatItems

etc

What I am trying to do is query the chats table to find all the chats that hold a participant by a passed in username string.

Here is what I have so far:

subscribeChats(username: string) {
return this.af.database.list('chats', {
query: {
orderByChild: 'participants',
equalTo: username, // How to check if participants contain username
}
});
}

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